I can't say I've ever 'mastered' a game, only to forget it later- the games I usually play aren't twitchy, typically, so once I remember the controls it's fine. Sometimes I think I'm not as good at Elite Beat Agents as I once was (rhythm titles are the only twitchy ones I get into), but then I remember the hours of sitting at the table, playing the same stage over & over for that perfect score, and I know it's not that I was ever that good- just that I got lucky on one run.
I did once randomly & briefly become good at a game. Columns was one of the first two game we got for the Genesis when I was a kid. I was never very good at it- maybe get to level 20ish before it was too fast to handle. Then I got my PlayStation & put the genesis aside for awhile. I come back to it years later- and suddenly I'm a savant. Level 70-90 is my new norm. I don't even know how I'm doing it, it just happens. After awhile of enjoying the sudden skill, I put the game away again. Went back it later, and it's gone. I'm still better than as a kid, but now I top out around level 50 or so. Really kind of weird. I haven't played it years again, perhaps I should try it and see where I sit now.