Google MentalPlex gives some funny error messages:
* Error 01: Brainwaves received in analog. Please re-think in digital.
* Error 005: Searching on this topic is prohibited under international law.
* Error 008: Interference detected. Remove aluminum foil and remote control devices.
* Error 8P: Unclear on whether your search is about money or monkeys. Please try again.
* Error 666: Multiple transmitters detected. Silence voices in your head and try again.
* Error: MentalPlex(tm) has determined that this is not your final answer. Please try again.
More Google April Fool's Day:
Google April Fool's Day 2002: PigeonRank
Google April Fool's Day 2004: Lunar Jobs
Google April Fool's Day 2005: Google Gulp
Google April Fool's Day 2006 - Take A Guess