Fontifying-errors make Emacs (mostly) unusable

[Thorsten Jolitz <tjolitz@googlemail.com>]

Hi List, 

while cloning an Emacs major mode for a special wiki syntax, I
experienced that errors related to fontifying really can become
show-stoppers.

Lets say, I'm modifying/adapting some face definitions for the new
mode, but do not recognize that one old definition is still used in e.g.
a parsing function. Then, evaluating the buffer, I get an error message
like:

,-----------------------------------------
| Symbol's function-definiton is void: ...
`-----------------------------------------

I can fix the error, but from that moment on Emacs core functionality is
blocked. M-x as well as M-: or e.g. C-x d do nothing anymore but giving
me the same error message like above (although the root-cause of the
error is already fixed).

Thats probably because the error appears in a (fontifying) hook
function thats called before any real action.

How can I get out of this 'trap' without restarting Emacs?

-- 
cheers,
Thorsten