This one is really inexcusable.  Anyone that studies Mark McGwire signatures (and I would have to hope that PSA would be in this category since they are getting paid for it) knows that around 1990, he changed his signature style from his mid-80's signature, with the thinner bottom loop of the "g", to the signature that he carried thru the 90's with the much more flamboyant "g". 

The issue with the ball in the subject is that it appears to be an attempt at a mid to late 80's signature, but was signed on a Gene Budig ball, which dates it to at least 1995, and then the inscription "STL25", which he used in 1998 during the home run chase.  Here are photos of this forgery:

Here is a photo of an authentic Mac signature from the 80's:

Here is the link to the auction:

http://www.ebay.com/itm/MARK-MCGWIRE-Signed-Autographed-Rawlings-OA...

For comparison purposes, here is an authentic 1998 Mac signature:

I see a lot of TPA errors, but this one is more bothersome because even a novice, with a little research, could have determined that this signature is nothing more than a bad forgery.