Emacs blew my Goddamned mind one day when I was using M-. to examine the Lisp implementations of various Emacs commands. One day I dug a little too far and tried to examine the source to what turned out to be an Emacs primitive -- but that didn't slow down M-., which happily jumped straight into the Emacs C source to show me the implementation of the primitive. (I had compiled this Emacs on my own, and apparently it remembered where it was compiled.)