entymologist's dilemma
You learn one new fact (e.g. the scientific name of an insect...) and an old fact pops out of your head.
The "entymologist's dilemma" occurred to me when I tried to recall the term for the roll-around cart that you use to work under a car...(a "creeper" by the way). (don't ask me what the term was that dropped out of my head...!)