I've had my 1D3, 24-70L, and 70-200/2.8L IS for over 2 years each, and they've been great. This month I sold my 70-200 and go the new 70-200/2.8L IS MkII and it's AMAZING!
I mainly leave that lens on my camera, then the other day I take it off and try to put on my 24-70L... it wont fit. It's almost like it's VERY tight after a bit of work I managed to get it attached and turned and locked. It took a lot of effort, but I was able to get the lens off. After doing this about 10 times, the lens was fine, attached and turned easy peasy (the new 70-200 would always fit perfectly.) I thought the problem was solved!
Today, I went to the zoo, took off my 24-70 and it came off nice and easy... I used the 70-200 for a couple hours then tried to change back to the 24-70 again... Sure enough, I couldn't get that sucker to turn. It pushed into the body but it took all my strength to get it on. Once again removing it and replacing it makes it smoother and smoother to turn.
I've looked at the threads and they seem fine. Has anybody heard of this before?
I don't have a Canon Service Centre anywhere with 2000km, so I will probably leave my 70-200 on and go down to a camera store tomorrow.