Installing a bunch of packages willy-nilly resulted in this configuration problem for me: starting slime resulted in this frustrating error message: Versions differ (2014…) slime vs (2009…) swank

It asks you whether to continue. If you say ‘n’, sorry, no slime for you ! If you say ‘y’, no fancy slime for you; you have to be content with the Comint buffer mode

If you want to fix this, you’re going to have to fix a possible mismatch somewhere.

In my case, I ran locate swank-loader.lisp and found two instances: one under /usr/share/common-lisp/... and the other under /usr/share/emacs24/site-lisp/..., you will probably have different paths but similarly conflicting.

Running dpkg -S on the first, I found it came from the cl-swank package, so I uninstalled it.

Now, slime refused to start because it was trying to load the missing swank-loader.lisp file.

Luckily, this was simply a matter of running M-x customize-variable swank-backend and set it to the remaining path (e.g. in my case, "/usr/share/emacs24/site-lisp/google/slime/swank-loader.lisp")

After this, M-x slime worked as usual!