The lens detection Microswitch
On some EOS bodies there is a small microswitch in the lensmount which must not be tripped by any attached lens or lens adapter in order for the camera body to work. If an adapted manual focus lens trips this microswitch, the camera body them looks for a valid EF series lens to be attached. If it doesn't see the correct electrical connection the the camera body thinks there is an error condition and will not operate. I've never been able to find an official list of which bodies have this switch and which don't.
The early 1D series bodies do, up to the 1D MkII. The 1D MkIII doesn't. I think the D30, D60 and 10D have it too (note that's the D30/D60 not the later 30D/60D). Some, possibly all, EOS film bodies have it. I know the EOS-3 does for example.
If you have one of the bodies with this microswitch and your adapted lens causes the camera to freeze you have to remove a small part of one of the bayonet flanges in order to avoid tripping the switch. Either that or do not lock the adapter completely in the EOS mount. If you don't fully rotate it so that it "clicks" into place it won't trip the switch. Of course the adapter is them not fully locked onto the camera, so you need to be careful not to accidentally allow the lens to part company with the camera!