If you use Emacs 23 and ido, and you see this error when you run ido-switch-buffer:

Error in post-command-hook: (wrong-type-argument sequencep t)

…you might have an outdated version of ido.el. You probably have version 1.56, from 2002, which is included in many Emacs packages. Details on the bug in this emacs.bugs thread.

Happily, the bug is fixed in the 2009 version of ido.el. Take a look at your Emacs package and see if it has that version, maybe in /usr/share/emacs/23.1/site-lisp or somewhere similar. If not, grab it and put it somewhere in your path so that it takes precedence over your package’s old version. Happy editing!