After qualifying, the FIA's scrutineering team found the DRS clearance on both Haas cars to be greater than the maximum allowed opening of 85mm.
The matter was referred to the stewards, who confirmed the usual penalty of disqualification for both cars.
Like its fellow competitors, Haas brought a new specification of rear wing to Monaco to cope with its huge downforce demands.
Haas explained that it had simply made an error with its new wing, which needed to be set up differently to ensure compliance with the rules, and accepted its responsibility for the mistake.
"The team explained that this was the consequence of an inadvertent error on their part in setting the wing flap gap," the stewards wrote in nearly identical statements covering the two cars.
"The wing used was a new design that was used for the first time in Monaco. The old design was set to be compliant with the regulations with the largest gap measured from the centre of the wing.
"Under the new design, the largest gap was at the extremities of the wing but the team had not trained its mechanics to set the gap per the new design, resulting in the non-compliance."
"The team suggested that there was no performance advantage gained by the non-compliance. Article 1.3.3 of the International Sporting Code makes it clear that this is irrelevant. The team was candid in admitting its error."
Lewis Hamilton was punished for a similar problem in sprint qualifying at the 2021 Brazilian Grand Prix, which led to the Mercedes driver losing his pole for the sprint and being demoted to the last place on the starting grid.
On Saturday, Magnussen qualified 15th in Q2 after being asked to abort his final flying lap, much to the Dane's anger.
Hulkenberg fared a little bit better in 12th, but also faced an earlier trip to the stewards for impeding the Alpine of Esteban Ocon in Q2, explaining his radio communication system didn't work so he was unaware of Ocon approaching.
The stewards decided not to take further action, because they had received multiple reports of radio issues across multiple teams and because Ocon pointed out he wasn't significantly impeded.
