The look on John Harbaugh’s face is etched in the minds of diehard Patriots fans.

It was a combination of confusion, anger, and helplessness. The Ravens coach was apoplectic as he watched his defense run around like chickens with their heads cut off as they tried to stop a Patriots offense that was marching down the field in an AFC divisional-round game on Jan. 10, 2015.

With the Patriots trailing, coach Bill Belichick employed a strategy that called for just four offensive linemen. Needing to have at least five players on the line, the team had running back Shane Vereen or tight end Michael Hoomanawanui, normally eligible receivers, check in as ineligible and take a spot on the line.

The Ravens continued to cover the “ineligible’’ receiver, which often left an eligible target open.

“It’s not something that anybody has ever seen done before,’’ said a furious Harbaugh after New England’s come-from-behind, 35-31 win. “They’re an illegal type of thing.’’

The truth is, it was legal (“Maybe those guys gotta study the rule book and figure it out,’’ was Tom Brady’s postgame quip). Harbaugh’s comment that it wasn’t “something that anybody has ever seen done before’’ was actually a compliment.

It was an example of how the Patriots stay ahead of the curve in today’s NFL. Because of the perception that the franchise doesn’t always play by the rules — the fallout from Spygate and Deflategate scandals — the Patriots are often criticized, rather than praised, for their innovativeness and attention to detail.

Shortly before the Ravens game, former Dolphins coach Don Shula perpetuated the perception when he referred to Belichick as Belicheat. The comment came across as petty from a man still bitter about Mark Henderson’s snowplow ride in 1982.

The “hey, they’re cheating’’ reaction popped up again in last season’s 28-21 opening victory over the Steelers.

During a crucial goal-line stand in the fourth quarter, the New England defensive line moved in unison at the behest of linebacker Jerod Mayo. The Steelers jumped and were nailed with a 5-yard false-start penalty.

After the game, Pittsburgh quarterback Ben Roethlisberger said the Patriots broke an “unwritten rule’’ and insinuated they were trying to simulate the snap count — which is illegal. The Patriots’ tactic, however, was perfectly legal.

“We don’t have to move at one time,’’ Belichick said the day after the game. “We’re not trying to simulate anything. We’re just trying to move the defense . . . Defenses have done that for, I don’t know, probably 75 years.’’

Keeping people on their toes is a Belichick specialty — and that’s whether he’s focused on his team or his next opponent.