bidding on the advertisers trademarks. For example you cannot bid on blockbusters name to send them sales since blockbuster is a trademark of blockbuster. Make sense?
It may also include not bidding on the trademark URL, mis-spellings of the trademark or using the trademark in your ad copy. Its best to check the terms and conditions of the program and when in doubt email the affiliate manager. The shortcoming is spending money on PPC and then having commissions reversed because you have violated terms.