Yeah I have the same problem sometimes, like today my warm up was horrible casue I was sick and haven;t played in two days. Terrible. Also I remember at least one instance when I literally forgot how to play a guitar in the middle of my practice, I don't even know how it is possible. I just couldn;t play anything any more, I forgot every scale and chord I know and evey time I tried to hit a fret I would be off by at least one. weirdest thing ever and I just set it down and twenty minutes later I was fine.

As for the F chord, I am glad I didn't really know about it until recently I have never had any trouble fretting it :D I had been playing a year and a half before I actually knew about it though so...