After changing Fluxbox workspaces (using the keyboard), the first key pressed is lost.
For example: In workspace 3, a terminal window is open. Then switch to the next workspace and then back (Ctrl+Alt+Left per default). Type "abc" - usually only "c" appears in the terminal window.
See Debian bug #612893, especially message 87: http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=612893#87
He describes it pretty well - if you manage to release the Ctrl and Alt keys before you've arrived on the new workspace, they're not released (race condition).
I'm using Fluxbox 1.3.2 on Fedora (fluxbox-1.3.2-4.fc17.x86_64) and I consider this to be a serious bug. Too many times (using vi), one of the key strokes switching to insert mode has been lost and suddenly D clears the current line.