There's a pretty major bug in the Feywild Wilderness, where if you kill the Grim Wood Walker rare, it doesn't give you credit for killing it (you still get the chest), but if you kill Vorkin's Ghost, which is in a completely different part of the map, it gives you credit both for Vorkin's Ghost AND for the Grim Wood Walker.

I call it a "major" but because it's not a "weird combination of circumstances causes unexpected behavior" bug but a flat out "someone messed up in coding this" bug.