> But the other important thing to remember is that Emacs has excellent undo. You don't need to ask users, "Do you want to paste register 'd' containing '...'?", because you can just paste it, and let the user undo it if they chose the wrong register.

With paste you can see what got pasted, so you've got a chance to realize it is not what you wanted.

But how about for copy? If you meant to copy into register 'r' but missed by a key and typed 't' would it be noticeable right away?

I don't use Emacs so don't know how its undo works, but when I use named registers in Vim it is often to hold something that I'm not going to paste for quite a while. By the time I notice the error it would be annoying to undo all the way back to the mis-copy.

If copying into the wrong register was common enough to need to be addressed, my first thought would be something like adding a status message pups up for a short time near the cursor that says something like "Copied to 't'".