An Invisibob is a special
avatar glitch from Gaia Online. This
avatar glitch was only possible on April Fools' Day of
2004.
Since the YGS, or "You got Served" boxes, exploded in your
avatar'
s face, it looked like there was soot on it.
The Administrators had planned for it to go away after 24 hours, but it didn't - they'd stay until you saved changes to your avatar. Thus, this led to
two types of rare avatars: the Sootfaces and Invisibobs.
An Invisibob was created by deleting your avatar data, then clicking on a YGS box. The resulting explosion glitched the avatar system and forced your avatar to go to the default base - a Soot-face and a pair of floating boxers.
...See one on Gaia Online, by doing a search in the memberlist for "Sevi Rais.".