Starcraft - I used to be fairly good at this game. Not amazing, but I could hold my own online and I played it for the first time in years a while ago and I am pretty bad at it now. I mainly blame the super Koreans who are constantly getting better at the game but still.
Super Mario Kart - When I was younger I could beat every circuit on 150cc but going back to this game now, I can't beat the Special Circuit anymore.
Counter Strike - Same thing as Starcraft. I used to be very good at this game but now I am substantially worse.
Granted if you stop doing something for years you will naturally get worse at it, so I am not too surprised but it still kind of sucks.