About `lookup-key'

[Xue Fuqiao]

In (info "(elisp) Functions for Key Lookup"):

  If the string or vector KEY is not a valid key sequence according
  to the prefix keys specified in KEYMAP, it must be "too long" and
  have extra events at the end that do not fit into a single key
  sequence.

It is the documentation of the function `lookup-key'.  I have a question: why 
must it be "too long"?  Can't it be a wrong key sequence, such as `C-!' in the 
global-map?

-- 
Best regards, Xue Fuqiao.
http://www.emacswiki.org/emacs/XueFuqiao