Personally I like this one:
P.S. Do not hotlink the cartoon without the site's permission please.
I knew it was true. XKCD #224
"We lost the documentation on quantum mechanics. You'll have to decode the regexes yourself."
I can't believe someone hasn't put this one:
alt text http://www.jeffiscool.com/pictures/Foxtrot/foxtrot_bluescreenofdeath.JPG
https://brainstuck.com/2008/08/08/t-c/
I'm guessing that this would probably never happen, as agreeing to the terms and conditions is more important than actually reading them (from the software publisher perspective).