Back in the day, when I was taking lessons at this little dump-y barn, there was a mare that unexpectedly had a baby.
I can't remember the exact details but I think it involved the basically wild mare staying in a trailer, within the pasture of a stallion, for few days and somehow getting bred...
Of course, the owners were super sketch - not to mention, who leaves a horse in a trailer for a few days, especially a mare in a stallion's pasture?? - so I have no doubt that it was the lack of proper prevention that caused that baby.
Luckily for the baby, it was a super flashy palomino appy so I bet it probably eventually got a good home. However, I can't vouch for that since the babeh and the still wild mama were moved out of that barn pretty soon after the baby was born.
That, however, is my only experience with an "accidental" baby.
Well, I guess there was the time at the place I used to board where a 3 year old stallion was let out with Lacey for a few days (without my knowledge, initially) because "he hadn't dropped" and "Lacey's not in heat, but a baby would do her good!" Thankfully, after I made a fuss, the two of them were separated and Lacey didn't become pregnant.
That was just plain stupidity at work.
Fabio - 13 year old Arabian/Lipizzan gelding
Rest peacefully, Lacey.