Don't try to mount EF-S lenses

[M_Six]

With a battery grip in place I can *almost* understand that.

[apersson850]

You have both on the same table, grab the camera and start mounting the lens. The 7D has a WFT-E5B mounted, so it's not too different in size, and mounting lenses is something I've done so many times that I feel I can do it in the sleep. Well, considering the result this particular time, it kind of was in the sleep...

As Forest Gump said: "Stupid is as stupid does".

[peety3]

With a battery grip in place I can *almost* understand that.

If you've picked these up with your eyes closed, there's a distinct feel difference in my opinion. If you grab it by the normal grip, your pinky probably feels a more distinct bump of the battery bay on a 7D or the noticeable separation between body and grip, your hand feels a subtle compressibility to the plastic underneath the grippy material, the thumbstick has a notably different surface (recessed with a notched ring on 7D, raised/gridded on 1Dx), and then there's the weight. If you grab it by the vertical grip, the 7D is rather unergonomic IMHO, there's no second thumbstick, and my fingers are always tempted to verify that the tripod screw is tightened.

If by chance you have a Really Right Stuff L-bracket on it, the 7D has a square Arca-Swiss base (which I hate) that gives it a weeble-wobble feel on a countertop, while the 1Dx has a long rectangular AS base that I find a lot more stable.

That said, sorry you bumped into this problem.