The worst, is going from any Halo to any other Halo.
The button layout, the feel of the game, and how hard the enemies are, all takes about an hour to get used to =|
I get the same problem in any CoD going to any other CoD.
For the week or two of Blops MP that I played, I kept trying to dropshot on the run, and was rewarded with my dying body bouncing off the ground...
Its also really inconvenient to go from one Fire Emblem to another, and be disoriented by the different stat coefficients, and how units behave.
I don't think I've ever used a skill I have acquired in one game, in a completely different game... After playing Halo for a while though, going to CoD makes me realize how OP shields are, lol.