There are true accidental Breedings, I know. If the neighbors stud gets out, runs through your fence and breeds your mare while your at work, fine. Or various other truly accidental situations that might happen, but having a stud on your property (of ANY age) requires appropriate fencing and precaution.
For example: my yearling/ two year old can't breed, he's not interested. RIGHT. Inless your yearling is being watched 24/7, he will at some point become interested. All it takes is a frisky teenaged colt and a cooperative mare with 5 minutes alone and PRESTO! "Dang, he WAS interested. I thought he would tell me first." Its like saying their 17 year old son isn't interested in the opposite sex because they havent caught him in bed with someone yet. Your colt will not WAIT for you to be there so he can have an audience the first time he "gets interested"
Separate your colts at weaning from ALL mares and fillies, behind stud proof fencing. If you a) don't have appropriate fencing, b) Don't have the money to geld or c) "I want to wait untill the bugs are gone", you need to put up the fencing, find the money to geld or give him to someone who will.
I'm so tired of people complaining that their colt got their yearling filly pregnant, their 12hh pony mare got bred by their "uninterested" 16hh 2 year old qh, or their badly built, cronically lame pasture puff got bred by their new yearling last year and now looks pregnant.