The Sign Change is not the most common card, but there is probably one in each deck. It can do any of the following:
-Change win conditions to the winner being the person with the fewest points instead of the most. This overlaps with Rule Card.
-Change all + sign on cards to - or *. Or / if you want to get REALLY crazy.
Some forms of Sign Change need to be carefully worded. For instance, a card which simply says "Any cards played to this one have their effect inverted" can and will invert things that were simply not meant to be inverted: Blast Cards, Creativity Contingent Cards, Blank White Cards...
If you want to keep all of your sanity, you'll consider those which can do this as Suck-worthy cards.