Sheepocalypse
So, I sign onto the server after the downtime all day, and there's no obvious file or chunk errors so I assume all is well. I sign out and sign back in and I see...what looks like a white firework. I go investigate...and I see THIS:
It was a sheepocalypse, like 40 of them spawned! I got over 80 wool for killing them. But that shouldn't happen, I've only seen that behavior in bad chunks....I don't see any bad chunks do I????? Turns out I DO!
It's kind of hard to see, but that's a legit "killed but regenerated" chunk right near my castle. Fortunately, it's minor and almost fits the area, but it's an interesting study in errors.