That's a sort of hard question to answer because any one punishment type might result in a lot of pain or discomfort one time, and not another. The most pain I've experienced is typically when something unusual has happened, perhaps something went wrong. Like a pulled muscle.
I've actually hurt myself as much as anyone else has. It happened during a self bondage session when I was a teenager, probably about 15 or 16 years old. If you read earlier entries you might recall I started experimenting with tying myself up at a rather young age, before high school, and before I even understood that bondage was a kink people practiced.
By the time I was well into high school I was also well into self bondage. I had read up on it some, gotten ideas and the like. My first experiment with it almost ended in complete disaster (being discovered by my parents) and taught me quite a bit. I fortunately got out of the tie before anyone came home.
The easiest way to do self bondage is with locks on some sort of timer, but I had not come that far, and didn't have the equipment available to me. So I made do with rope. I either incorporated a slip knot of some sort, a hook grab onto a loop and pull the rope apart, or a knife of some sort. Playing with how far I had to go to get to the knife, and how I had to struggle to get it were one part of the play.
I always tied myself up naked. It was a sexual experience, totally. I was aroused, wet, and afterward I always masturbated.

I had a good long length of soft cotton rope. I had learned not to use nylon and other types of rope that were coarse and hard and left abrasions. Not because I minded the discomfort (I sort of liked it, actually), but because it left physical marks that were hard to explain. This cotton rope worked really well; it didn't leave marks (well, not bad ones) and would tighten nicely, even stretching a bit.
This particular scene I did in the garage. The garage was cold and dark, not a comfortable place to work at all. But it had the unique quality of being outside the house, and thus making me feel even more vulnerable and exposed while naked, but still being closed so no one could see me. I would dream, fantasize about being kidnapped and held in a garage or basement.
I entered the garage, turned on the dim light and removed my clothes, folding them neatly on the side. When I was naked I began to feel the amazing arousal and vulnerability, exactly what I liked. It was cold in the garage, and I shivered a bit.

I inserted a homemade dildo I made out of some molded play-dough with a lubed condom over it, and duct taped it in place. I felt filled, and it was uncomfortable. Just what I wanted. I wanted the feeling of being kidnapped and raped against my will, completely helpless.
Two short pieces of rope tied around each breast, causing them to bulge out. My breasts were still something of a novelty to me, and this part made me feel especially sexual.
Two short pieces of rope tied around each breast, causing them to bulge out. My breasts were still something of a novelty to me, and this part made me feel especially sexual.
Pulling on the garage door opener emergency release disconnected the opener from the actual door. The mechanism would still slide back and forth on it's rail, but it wouldn't pull on or open the door. Pressing on a garage door remote reeled the opener mechanism all the way back, as if the door were open.
I then took the end of a piece of rope cut to a carefully measured length and tied it to the catch of the opener slide that normally connected to the door. Throwing the other side of the rope over a rafter, it dangled down. That end of the rope had a cinch tie prepared on it, ready for my wrists.

My nipples were hard at this point; I played with them just a bit. Part of it was the cold of the garage, but part was the incredible situation I was putting myself into.
So, I was on my knees in the middle of the garage with the cinch knot behind me on the concrete floor. I put my arms behind my back, wrists together, and wriggled into the cinch knot. The homemade dildo stuck up inside me was uncomfortable, poking into my cervix as I wriggled around.
The whole thing was amazingly delightful. The only thing I wished I could do is provide some masturbation technique while tied up like that. I wanted to feel that dildo shoving in and out of my cunt. I wanted the sensations of being tied up helpless and raped.
Now came the ultimate goal of the scene.
On the floor next to my hands was the garage door remote. I leaned back, and got it into my hands. I sucked in a few breaths through my nose, feeling the cold on my naked skin, the tensions of what I was going to do filling my body. I savored that moment, and imagined I was a helpless victim.
Then I pressed the button on the remote.

That was enough. The wrist restraints were tight and secure, I was pulled up off the floor, helpless. But the opener wasn't done. It kept going. I had measured everything carefully, except... the length of the track for the opener. It kept pulling the rope further, my arms raising up behind me, higher and higher into a full strappado position. The wrist pain was the least of my problems; my shoulder and upper arms were twisting and pulling and the muscles began to scream in protest. Muscles twisted and wrenched in ways they weren't designed to, and I let out a muffled scream.
Muffled because the gag I had applied did a really good job. Good thing, too, because I could not help screaming and if someone had heard me they would have actually tried to help, revealing the completely inappropriate activity in which I was engaged.
I had only misjudged the length of the garage door opener pull by about a foot, but that was enough. With a creak, a grinding and mechanical straining noise, the opener pulled my rather skinny body off the ground. When it stopped, my knees were dangling about 2 inches above the ground, though it could have been 20 feet; it didn't matter to me. I was suspended in strappado arms twisted behind me, my body leaning forward slightly, my young but large breasts dangling forward.

It was time to press the garage door remote button, and let myself down, pronto.
Except that little remote wasn't in my hands any more. The strain of the cinch tie and pain of the strappado had weakened my grip and it was on the floor behind me. I was stuck. I didn't know it at the time, but the strappado position I had placed myself into was an ancient medieval torture technique. Had I realized this I might have thought twice about it.
I hung with my arms almost vertical up behind me, the cinch tie cutting off circulation to my hands. And trust me, it hurt. I am surprised I didn't dislocate something. Tears streamed down my face, every struggling movement I made increased the torture. I didn't know what to do and started to panic. It was two hours before my parents came home and I could not face hanging in the strappado position that long, and then be discovered when the garage door failed to go up and they investigated, only to find their 15 year old daughter tied and suffering, dangling from a rope hanging from a rafter.
OK, so there I was hanging, slowly swinging back and forth, naked and cold and in pain, arms pulled painfully behind me and the remote on the floor.
My knees were only a couple of inches from the floor, too. The pain in my back and shoulders and arms was making me sob, I could tell there was some damage in my left shoulder. If only I could reach the floor and support some of my weight, at least I would be slightly more comfortable.

The bouncing motion was accomplished by my leaning forward as much as I could, lessening the angle at which my arms extended upward in relation to my body, and then suddenly releasing, causing my body to drop down slightly. Each drop caused me to scream into the gag again, and sweat was streaming down my face, in spite of the cold in the garage.
The third time I dropped, the garage door opener broke. One of its supports yanked free and I was dropped to the floor with the opener hanging above me, only partially suspended by one support. I rested for a bit, letting the pain subside some, and then slowly wriggled free.
When my parents came home I was mostly recovered, though I had to tell them that I had twisted my arm (my left shoulder was a lot worse than my right) when trying to climb up to the garage rafters to get some old ski equipment. I had slipped, grabbed the opener as I went down, and twisted my arm in addition to pulling the opener loose. A lame excuse, but the only one I could come up with. I went to the doc, who gave me anti-inflammatory meds and a sling. I wore the sling for a two weeks.
That was actually the point when I began to realize I needed a bondage partner. I couldn't keep doing this to myself and taking the risks.
The other time I can tell you about when I had significant pain during a session was with a boyfriend just before Jason. His name was Ben. We were only together about 6 months. He was a good bondage partner because he was really into it, he dominated me and was very skilled. But... he wasn't always as aware as he should have been.

I was crying, with snot running down my nose and mixing with the foaming saliva around my ball gag. I tried to tell him something was wrong, that I thought the nipples were getting hurt too badly. It took a while for him to realize (he was too busy turning the underside of my boobs into angry red stripes), but he finally undid me.

When I was finally free, I just curled up and cried. I think my nipples hurt for several days after that and were incredibly sensitive for a couple of weeks.
There are other forms of pain that I've experienced, some quite deep. One of the most disturbing types of pain is from slow, constant, boring situations that just go on and on. It's one reason I hate the wooden pony so much. The pain is bad, it starts off uncomfortable and just very slowly gets worse and worse. You rock and wriggle, trying to get some relief, which works for a bit but after a while nothing helps any more.

It's the psychological aspect of this punishment that gets to me. The slow, relentless torture that starts as discomfort and slowly grows to agony with no immediate end in sight.
These days I suffer on the wooden pony about once a month or so. Sessions are as short as a half hour (pretty uncomfortable) to four hours (agonizing). Jason's pony is a simple saw horse. The top rail upon which I sit is thinner than a 2x4, and tends to sink deep between my labia lips, causing a lot of deep pain.
I gladly suffer for Jason. He is my owner, my husband, my lover, my master. I am his to do with as he pleases, and if he pleases to make me suffer, then I will suffer for him. It doesn't mean I always like it. I hate the wooden pony.