People have been reporting, on various message forums, that the Viewtiful Joe 2 demo included with Sony's Holiday 2004 PlayStation Underground demo disc WILL ERASE MEMORY CARDS.

Apparently, if you play through the demo of Viewtiful Joe 2 that's on the disc, AND have a memory card inserted in the machine, the game will ERASE the memory card once you complete the first area.

Here is Sony's official response:

Dear PlayStation(R) Underground Member,

You are scheduled to receive the Holiday 2004 Demo Disc
including the playable Viewtiful Joe 2 – please take note.
It has just been brought to our attention that there is a
glitch in this demo which will erase all of your saved files
from your Memory Card(s). If you have not yet played the
Viewtiful Joe 2 demo, please remove your Memory Card(s)
from your PlayStation(R) 2 computer entertainment system
before you load the Holiday 2004 Demo Disc.

Sony Computer Entertainment America would like to express
our sincerest apologies for any inconvenience this may have
caused you.

FYI–the disc contains playable demos of Jak 3, Ratchet & Clank UYA, Star Wars: Battlefront, Viewtiful Joe, and ATV Off-Road Fury 3.