it is the one where they cut your mana when a monster dies. the description specifically says that 15% mana is cut when a monster dies, and this works properly when you kill a monster with a special skill - immediately all heroes get their mana cut. however, when you kill a monster with tiles, it doesnt cut the mana right after death, it waits for all tile cascades, which might fill your heroes’ mana, and THEN cuts the mana.
either make it consistent with the description, and apply mana cut right after the mob dies. or change the description and apply the cut at the end of the round of specials and tile moves. not “we take your mana when it hurts you most, bugger the description”!