(Another tool posting. Apologies to the pure basketball types, but this problem and its fix are impossible to find on the Internets so I wanted to document it for others.)
If you use the Superior Lisp Interaction Mode for Emacs (SLIME), you may get an error when starting up Slime that says "Slime and Swank versions differ, continue?" Answering yes to the prompt usually continues without any problems, but it's still annoying.
The bug is caused by having multiple copies of Slime on your machine. To fix it, you need to find and remove all older versions of Slime. (Searching your file system for "slime.el" is a simple way to find all the installed copies of Slime.) You might also need to clean up your .emacs file to not load (or include on the load-path) those old versions.
However, the problem will persist until the "slime.el" file is recompiled. To force this, find the "slime.elc" file in the remaining installation of Slime and remove it. The next time you start Slime in a new Emacs the file will be recompiled and the error should go away.