That depends on what I am doing. For something like rope, where the confidence is in my own technical ability, if I lose confidence mid scene, I undo the tie I was working on, then either restart it, or switch to something I know I can do.
For times where the confidence issue is more in how the bottom is handling things, like an impact scene, I will pause to check in. If they're reaction does not restore confidence, I end the scene.
When I do new things, it is almost never in the context of a scene, but rather more of a workshop environment, where the goal is for me and/or my partner to get experience doing a thing. Since we are both in a learning headspace here, there is a lot more room to safely make mistakes.