(Supposedly) standing in between 4 drains leads to XP duplication (slow increase, the more XP the more you dupe).

Demonstration in Revelations pack 2.3.0 SMP: https://www.youtube.com/watch?v=jrQm7kzQmms&t=4m57s

It was problem with shower and small amount of leftovers in tank (nothing to do with drains). Anyway, fixed.