That is the bug "that doesn't exist". For some reason, when you level and
before you max your skill your fizzle rate increases. THis is obvious for
your new songs/spells, but it definitly occurs for songs/spells you have
been casting for a long time with no trouble.

I noticed it while playing my cleric and asked my roomate and friends to
watch for it too. They all see the same effect with their casters. It
seems to be the same for Elfson (my bard) though it doesnt bother me as much
since all you lose is time (not mana)...