commented on Savage Love Letter of the Day: I've Looked At Accidental Anal From Both Sides Now
It happened to me and my wife once. We were both very drunk at the time and going at it with corresponding abandon. We weren't doing it missionary--it was "doggy style" (ugh, is there a better name for that position, BTW?). I'm of average endowment (six inches). She said "ow" and we stopped immediately.
That's it for accidental penetration. But I've poked her anus without penetration accidentally several times--at the start of sex, aiming for her vagina and missing, in the dark. The head of my dick isn't sensitive enough to really know what I'm poking at (thanks, circumcision!).