Or the opponent has untreated ADHD causing them to keep losing their journal/spellbook, and, after searching their house, cleaning it, fixing a drawer slide, and picking up the lute for the fifth time in two years, they have finally found 4/5 of their lvl1 spellbooks.
All of the spells contained therein, however, are in various states of completion and riddled with attention-related errors, so, they may be able to put together a magic missile spell but it is quite likely to have unintended consequences like a half-eaten bagel materializing in the target's pocket and spectators suddenly having minty-fresh breath.
I can only think of two plausible interpretations of this concept.
Mage is broke as hell and cobbled together his own grimoire by hand-copying spells from the college library, friends, and the occasional dungeon find. Yeah, a few occult incantations slipped in there, but you'd hardly notice for all the doodles and random "todo" lists from years ago.
