We had many sayings in my Naval Photography career. One of my favorite was, "To ASSUME makes an ASS out of U and Me!"
I was guilty of that on my trip to Europe and Istanbul. I have a Seagull Right Angle Viewfinder which I have successfully used on all of my previous Canon DSLR cameras. I used this finder on many different model DSLR cameas from the D60 through the 40D. So I naturally (and stupidly) assumed that it would work on my 7D cameras also. It did not. I learned that the first night in Venice when I tried to attach the right angle finder to the 7D viewfinder. It was the wrong size, somewhat smaller.
So the moral of this story is "Don't ASSUME that a piece of gear will work unless you have tried the gear with your equipment. That is unless you want to be made an ass of like I was...