I didn't directly witness the effects of this, probably because my difficulty was set to low and I never fought anyone, but in the end game report, the mana bar for one of the AI civilizations (Gilden) jumped to an obviously erroneous high number.  And if I'm to believe the graph, he made use of it all somehow...

https://dl.dropbox.com/u/1190647/images/elemental_mana.png (full screencap)

EDIT: I should say I have a savefile from the END of that game, if there is anything that might be gleaned from it, but the bug occurred early enough that I doubt it.

https://www.dropbox.com/s/91bwx58r68aehsn/EFE-ManaBugEndgame.EleSav