[anonymity]

When there is a spell on the script that cannot be used (potential targets are out of range, or immune, or the buff has already been applied), the AI will pick another spell to replace it in the script. If your mage is holding gems, it can be a spell that uses gems. I've seen Arrow Fend casted by mage-thugs who were scripted for Blessing because all sacreds on the battlefield had already been blessed.

The lesson here is don't give mages gems they can't use. It's better to have a scout sneak alongside them holding gems, and you can transfer between when they're needed.