Happened to me just a little while ago, actually. For seven and a half years I've struggled to complete Final Fantasy VII (as I've gotten older I've had more patience to train my characters), and I've only managed to battle Safer Sephiroth once (this was about five and a half years ago) and he managed to kick my ass without a problem. Fast forward five years and a countless number of games I finally managed to get back to the final few bosses...and on my first try was defeated by Bizzaro Sephiroth. I leave it for about six months until about three hours ago where I decided to play it again, considering I was struggling with the final battle in Final Fantasy IX (I have no luck with these final battles...). First try defeat JENOVA easily, Bizzaro Sephiroth took me five minutes. However the battle with Safer Sephiroth took me fifteen minutes, twelve pheonix downs, five Super Nova's (which contributed to the fifteen minutes with it's over the top cinematics)...however it all ended with a Cid attack and my long struggle was over. Sorry for the little story there.
Another game that made be take a break would be Vice City. It took me at least thirteen attempts, and a two month break from the game, to beat Hilary in the Club Malibu mission. Played it again a few months ago and completed it first time no problem. I'm currently hoping this works with a mission in Vice City Stories (Light My Pyre, I think it's called).