I'm a native speaker. There is this sentence "an epiphany a long time in the making." Now it's technically an incorrect use of the word "epiphany," which is essentially something sudden. But what kind of mistake is it? What is the specific term for that kind of mistake? It's not a malapropism or a solecism. The closest thing is a "yogi berism" (No one goes there nowadays, it's too crowded). It's a phrase that's inherently contradictory. My question is, is there a name for this? I don't think it qualifies as an antithesis. Any linguists out there who could help?
What you’re describing is an oxymoron.
oxymoron noun A figure of speech in which apparently contradictory terms appear in conjunction (e.g. faith unfaithful kept him falsely true). - ODO
As others have cited, there isn’t necessarily a contradiction between a sudden realisation and an extended prelude to that realisation.
Epiphany Literary Terms
Amy has been smoking for fifteen years. It isn’t until she gives birth to her daughter that she has a moment of revelation [epiphany]: she has to quit.
This is a more predictable epiphany, the moment of birth.
Your cited sentence containing "an epiphany a long time in the making" is grammatical and no mistake. Predictable in the reference implies events leading up the the 'aha' moment