Placing blocks in this configuration (with the XP shower attached directly to the tank on the upper right):

causes XP to go up indefinitely:

I started with about 15 XP in the tank. Seems standing further away from the shower while remaining on the XP drain exacerbates the issue.

This is in version 1.8 with the latest FTB Revelations modpack.

Seems the issue is caused by something else in conjunction with OpenBlocks, so opening an issue here is probably wrong.

It's interaction with Clumps. Still investigating.

Ok, found it: when looking for candidate orbs for merging, Clumps does not check Entity.isDead, so orb can be used even after it was picked by player.