The New England Patriots, one of the two or three best teams in the last five years, have been accused of stealing signals from the other team with a video camera.
Femember when the NFL changed the rules to allow a radio link from the quarterback's helmet to the sidelines? A smart team could not only eavesdrop on the other team, but selectively jam the signal when it would be most critical. The rules said that if one team's radio link didn't work, the other team had to turn its off, but that's a minor consideration if you know it's coming.