Not sure if this is really a bug but in the Antique Area Level 4 you can spawn unlimited Ancient Guards.

The 3 wall chests where you can obtain the navstone plates are intially locked. If you try to pick the lock directly (without a lockpick) and fail, a group of Ancient Guards is spawned. Each chest spawns a different group.

Spawning in Ambermoon means just show them on the map. If you kill a monster group, the game automatically hides the monster on the map.

So if the guards spawn and you kill them, you get the exp and they are gone. However if you try to pick the lock again and fail again, the monster group is shown again (aka respawn). You can repeat this unlimited. So this is basically a source of infinite exp.

Normally such event chains would disable this after the monsters were spawned once, which is of course possible.

What might be a bug in relation to this: If you failed the lockpick and the monster is already there and you approach the chest without killing it, the message ("monsters appear near you") is shown again but there is no additional monster, as it is already shown. This is more obvious if you lure the spawned guards to a different room and then try to lockpick the same chest and fail. There will be no monster in the room even though the message states it.