Sometimes even after feeling prepared I suddenly feel like maybe it isn't the time. For various reasons. I'm not warmed up enough I will still try and I usually eventually relax. But there have been times where it just didn't feel good so moved on to something else.
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.
I have been really hesitant to do anal with my current partner. I am not really sure as to why this is. I have engaged in anal sex with prior partners and actually enjoy anal sex so it isn't fear that it will hurt, I think it has to do with his whole attitude about it. Long story short I let him years ago put a finger there and for weeks after he kept laughing and saying "you let me stick me a finger in your ass." I think his whole acting immature about that kinda put me off on the whole idea.