A problem I sometimes have is that someone will suggest that I do something uncomfortable - physically or emotionally, whichever - and they'll acknowledge that it's not going to feel good.
And then they won't follow up by explaining exactly how much and in what way it's supposed to hurt, let alone why or how to mitigate that.
Look: if you're going for a run, you might get a stitch in your side, your legs might cramp, your lungs might burn. If you feel a sudden stabbing pain in your foot, that's not the run. You've stepped on something sharp and you need to stop and call for a ride, not tough it out. If you've been told that running might hurt and that's a good thing, a sign of weakness leaving the body or whatever, and there's any ambiguity between "stepped on something sharp" and "this is how it's supposed to hurt" - are you supposed to have blisters or do you need better-fitting shoes? Are you supposed to have a headache or do you need to drink way more water than you have been? Is a sunburn just part of a healthy experience that means you're making vitamin D, or should you be wearing sunscreen next time? - then you're blundering into a problem that you would have noticed and solved if you had encountered it by sitting on the couch.
If you're apologizing for something you've done wrong, you might feel guilty, stared-at, awkward, resentful. If you feel like the walls are closing in and you're going to puke, then you have a more serious problem and might want to get that looked into, not chalk it up to "well, acknowledging that you've made a mistake can be painful". If you're apologizing to someone who's abusing you, then how much it hurts is a red flag. If you're in a badly facilitated therapeutic or moderated conversational environment, and you don't know how much it's supposed to hurt, then when it hurts more than it's supposed to you won't notice that you need to fix or leave the setup. If you're apologizing for something that isn't wrong, because you've been indoctrinated in a normset that's unhealthy about what things are violations, then by letting that normset also tell you that it's supposed to feel bad you're losing a signal that could tell you to bail.
Even things you aren't actively doing can have this issue: it took me a long time to be really sure that most people don't experience pain when they have their blood pressure taken, because every time I complained about it, an indifferent nurse would say "yep, lots of pressure, mm-hm, it squeezes pretty tight" and not "actually, while you're unlikely to be in medical danger, that is not typical; we might make different tradeoffs about how often to recommend this test if it hurt everyone like it does you".*
And if you don't know how something is supposed to hurt, and know that you don't know it, you will need some risk-aversion there to avoid blundering into sharp objects and mental health triggers and cost-benefit analyses that were not designed for how you're put together. So this can cut off opportunities to try things, the opposite problem from trying too hard at something that isn't working.
If you are recommending that people do something that might be uncomfortable or painful, tell them what normal tolerances are, and what the things that might be causing abnormal responses might be. Demand this of people telling you to do uncomfortable or painful things too. Pain responses have a purpose; ignoring them outright for the duration of an activity which flirts with the damage that aversion is meant to prevent is insane.
*If you have this problem and might give birth and might have an epidural while you do, have them put the blood pressure cuff on your numb leg. Hat tip to Swimmer963.