Create a distinction between low-stress exercises and high-stress exercises. The former can be done to failure; the latter have to be stopped as soon as your form starts to break down. The former include a lot of body-weight exercises - push-ups, sit-ups, flutter kicks, planks, etc. The latter is any exercise where a breakdown in form is actively dangerous - deadlift, snatch, clean and jerk, squats, and so on.

Do the former all you want; the worst you're going to get is some soreness and maybe muscle strain. The latter will tear things.