Using the starfish trinket while using a self -> harm (Or any other damage type) spell from Ars Nouveau causes the xp gain on enemy hit to proc, allowing for infinite xp to be gained.

Probably fun to leave in. :P
Even if preventing self inflicted damage from giving exp you could do something similar with two players.
Even if limiting it to damaging non-players, you can still just make a mob farm and hit the mobs.

This exploit is also possible with the Target Dummy from the MmmMmmMmmMmm mod, that might be a bit too strong 😄