AGNES: Tilly, shoot them with a magic missile.
TILLY: I can’t.
AGNES: What do you mean you can’t?
TILLY: I forgot the spell.
AGNES: What do you mean you forgot the spell?
TILLY: It’s a thing. It’s not going to help us.
Click the above image of RHIALTO THE MARVELLOUS to read an informative blog post from The Evil GM on Jack Vance’s influence on D&D’s magic system (Vancian magic.) The basic theory behind this is that a wizard’s spells are so complex (s)he can only keep them in his/her memory until (s)he uses them, after which the spells are completely forgotten and must be re-learned. It is, indeed, a thing.
Also, the song “The Dying Earth” by the band “Wizard Eye.” Also a thing.