This is how I can reproduce this bug:

go to a myspace page with a flash music player plugin
click on a song to play it
banish the mouse
press the key that's bound to "conkeror -f unfocus" in the window manager (I have to do this twice for it
to work)
open the minibuffer with M-x (it will not respond to the text you type - maybe that's a separate bug?)
abort the minibuffer with C-g
press the key that's bound to "conkeror -f unfocus" in the window manager

then this message is displayed in the minibuffer:
call interactively: [Exception..."component returned failure code: 0x80004002 (NS_NOINTERFACE)
[nslInterfaceRequestor.getInterface]" nsresult: "0x80004002 (NS_NOINTERFACE)" location: JS frame ::
chrome://conkeror-mod...

Now conkeror stops responding to all keyboard input until I click somewhere in the conkeror window, or
sometimes switching to another window and back fixes it.

I can reproduce the bug on at least two different myspace pages.
I couldn't seem to reproduce the bug on youtube