A five-gallon hollow floating sphere of energy appears. This sphere can be moved by the will of the Magic User at her running speed, and exists for the MU’s level x 10 minutes. (So a level six MU can conjure the sphere for an hour.)When the spell is memorized for the first time, everything seems good until the Magic User decides to cast the spell. They will find that they rather prefer having the spell memorized for later than using it now, and won’t cast it.
This sphere has “jaws” as it were: the Magic User can will a variable semi-circle opening to appear or disappear either on the top of bottom of the sphere. This opening can be used to fill the sphere with items or liquid, and can also be used to grasp large items. Their closing will not crush anything, but will hold up to 200lb objects tightly enough to pick them up and move them. When closed completely the sphere is air- and water-proof.
Xena was not the magic user who originally crafted this spell, but rather his cat whose ability to grasp items in her mouth was the inspiration for his research.
To the character, the desire to keep the spell memorized “for later” will be stronger than any want, need, or coercion to cast it.
Story-wise, the spell is a sentient memetic virus of sorts, one that once memorized cannot be unmemorized because its desire to live (aka, stay memorized) is too strong.
Mechanically, the player just lost a first-level spell slot. If they memorize it again, they will lose that one too. Each spell is independently willful.
There is a plus side to this, however. Unbeknown to anyone living (except maybe one of the players’ antagonists), the spell’s will is so strong that even a changeling, once it has changed into a magic-user who has this spell memorize, will find itself itself unable to change again. The changeling will effectively become the magic user, and the two of them won’t even be able to tell each other apart.
What any person decides to do in such a situation depends on how fully they’ve given themselves into their magical studies and desire for power. I’ve written up one option, I’m sure you and your players can think of more.