When my partner and I started getting into anal, we didn't know that warming-up was an absolute MUST. I'd had anal with my exes, but they were average size and I could handle that without warming-up first.
I knew when we started talking about trying anal that my partner was well endowed, but I didn't know that it made any difference... Until we actually tried anal together. Because I was naive (despite having some experience) and he'd never tried anal before, we just kinda rushed into it without proper preparation. Needless to say, it hurt like hell, which put a damper on things. We decided we didn't really want to try it after all.
Thankfully, we learned about warming-up, and that made the experience(s) way more enjoyable.
So, yes, there were a few times where we were all ready to go and suddenly decided not to. Now that we know what we're doing, that never happens.