By a stupid mistake, I thought I was holding a 7D and attempted to put the EF-S 17-55 mm f/2.8 IS USM on the camera. But it was the 1DX I had in my hand. The EF-S lens doesn't fit, of course, but the problem was that the rubber ring at the rear of the lens, that ring which prevents mounting on an EF lenses only body, it opened the latch for the focusing screen. The screen frame opened, the screen fell out and got scratched.

Not that it's very expensive to get a new one, but quite unnecessary. So don't mess with EF-S lenses near your 1DX!