In 2015, ESPN reported that the New England Patriots had long had a reputation in the NFL for cheating. A former member of the NFL competition committee told ESPN that from 2001-2006, the committee spent much time "discussing ways in which the Patriots cheated."
The biggest cheating scandal was "Spygate," in which the Patriots filmed opponents' offensive and defensive play signals to study them. The Patriots would recognize signs during the games and relay them through headsets.
Though the Patriots had long been accused of filming opponents' signals, the scheme truly came to the surface when the New York Jets caught them in the act. Then-Jets head coach Eric Mangini, a former Patriots coach, knew of the scheme and told security to be on alert. The NFL investigated the Patriots, eventually destroying tapes the Patriots had of opponents' signals. The Patriots were docked a first-round pick and fined $250,000. Belichick was fined $500,000. He later said he misinterpreted the rules.
The next biggest scandal was "Deflategate," in which the Patriots were accused of deflating footballs before the 2014 AFC Championship. The Indianapolis Colts discovered the Patriots balls were below the legal PSI limit. The scandal included damning texts between Patriots employees about handling the balls, Tom Brady destroying his cell phone before handing it to NFL investigators, and a seemingly never-ending court battle. It eventually ended with Brady serving a four-game suspension, the Patriots being fined $1 million, and losing a first- and fourth-round draft pick.
According to ESPN, former Patriots staffers would admit to some methods of cheating during the Spygate years. For instance, they would steal play sheets from the opponents' locker room. The practice became so widely known that opponents started putting fake play sheets out.
Opponents also said their headsets would get scrambled when they played at Gillette Stadium. One of the more recent accusations came from Steelers head coach Mike Tomlin, who said in 2015, the Steelers headsets were tuned to a local radio station. According to NFL rules, when one team's headsets fail, the other team is supposed to abandon using theirs. Just as the Patriots were going to be told to stop using theirs, the Steelers headsets began working again. "That's always the case [at Gillette Stadium]", Tomlin said.
There were "regular rumors" that the Patriots filmed the Rams walk-through for the 2001 Super Bowl, which the Patriots won in an upset.