I don't know if my butthole is just especially resilient, but surprise anal has never happened to me. The penis has slipped TO my butthole, but it takes a lot more work to get it in there than just continuing the pace of sex.
The sum of my experience is: penis slides, makes contact with butthole, a couple thrusts trying to keep up the pace, butthole refuses to yield and no progress can be made, penis is manually reintroduced to vag. Which leads me to believe that a lot of men who "accidentally" wind up in the butt, were actually TRYING to get in the butt.
Though my husband was RIDICULOUSLY girthy, which could have been a factor. It was hard enough to get it in my butt when we were trying to have anal and I was actually relaxed.