I have ten Slime Hutches, almost all of them housing 20 slimes each, but for some reason every now and then I'll find a slime behind one of my hutches (always the same one), or even outside of the farm bounds. What's up with that? Is it a bug? Feature? Can I prevent it from happening?
According to the wiki:
It is also important to note that once the hutch has more than 5 Slimes, there is a 3.5% chance per night per slime to escape and disappear.
I'm guessing that's why you see a loose slime from time to time. As for why they sometimes appear outside the boundaries of the farm, I'd guess that's a legitimate bug (I've seen screenshots of other similar problems, like the player's pet being out of bounds).
Alright, I think I know what causes it. Sometimes a slime will glitch and get stuck inside the wall of a hutch, and this apparently causes the game to bug and put the slime somewhere else entirely. I recently noticed a pink slime being stuck in its hutch's wall, and later the same day, I found it freely roaming around my farm. When I went back to check in the pink slimes hutch, there was no slime stuck in the wall anymore.
Sadly, there's no way to prevent this from happening, so here's hoping it gets fixed in 1.1.