.. that you cannot play a Freak Drive if it is the card you draw to replace a "Do not replace until end of action" action card (i.e. Gramle, Bum's Rush or New Management). The reason is that the replacing the card played earlier (and you weren't allowed to redraw immediately), marks the end of the action, and you're simply not allowed to play action modifiers after the action ends.
Reference: VtES Usenet Newsgroup
Remark: This one's a classic, not really something that came up lately in the newsgroup or in my playgroup.