Keep in mind that yellow (power) pips count for two regular pips. So even an 8 pip spell will use only 4 power pips. Keep an eye on the color of your pips. Might not be a glitch at all.
Also, keep in mind that some of the bosses have insane cheats. I just remembered a card that reduces the pip usage for a spell. I think it's a treasure card. Maybe that card was attached.