Vimperator uses keybindings like ctrl+o for go-back-one-page-in-history.
Some sites on teh interwebs try to be more accessible by binding keyboard commands to specific actions, for example Wikipedia (and every other wikimedia site I have encountered) maps
ctrl+o to the login command (or at least something that requires a login).
So, I'm browsing happily along, and happen to follow a link to Wikipedia. If I want to go back a page, vimperator has trained me to ctrl+o. However, this keystroke is captured before vimperator can do anything with it, and I am taken to the Wikipedia login page. Do. Not. Want.