I don't think anyone can unintentionally be fisted lol. That kind of stuff takes a lot of build up.
Honestly, I've tried. More than once, just to see if I could experience it. And nope, not possible for me and my body. I can do four fingers at most, anything beyond that, even with a ton of lube and hours of prepping and stretch just feels like I'm getting torn apart.
Not a fan, no thank you.
I have however fisted my husbands ass, and using him like my own personal sock puppet was a delightfully amusing situation.