But it wasn't just the travelling Eagles fans who were frustrated by the incident, as Manchester United supporters took to social media to compare Johnstone's tackle to Casemiro's red card offence.
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Pedro Neto had burst into the Palace box in added time of the second half, with Wolves looking to double their 1-0 lead.
But Johnstone was clearly desperate to keep his side's hopes in the clash alive, as he went in on Neto studs up, crashing into his shins and leaving him rolling towards the Wolves fans behind the goal.
The referee immediately pointed to the spot and gave the Palace glovesman a yellow card, before Ruben Neves netted Wolves' penalty, sealing a 2-0 win in their bid to stay in the Premier League.
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But Johnstone's challenge clearly reminded Man United fans watching of Casemiro's red card against Southampton, which he received for a sliding challenge on Carlos Alcaraz.
The midfielder had originally been given a yellow card, only for it to be upgraded to a red after a VAR check - something Johnstone escaped at Molineux on Tuesday night.
And United supporters made their thoughts on the inconsistency of refereeing clear.
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"If his name was Casemiro instead of Sam Johnstone he'd be on a eight-year ban and a 12-month stretch in Alcatraz," one irritated Man United fan said.
"Casemiro got a red for the same challenge," another fan said, clearly believing his midfielder had been given harsher treatment.
Meanwhile, another said: "Casemiro also got the ball but I guess it's just United that only get the decisions… Both are still reds by the way. It's reckless."
"The inconsistency is mind boggling!! Can someone explain to me how Casemiro gets a red for almost the exact same type of challenge and he gets a yellow????" another fan said.
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A fifth simply asked: "How is that not a red card for Johnstone? Casemiro was red carded for less."
Casemiro's red card against Southampton marked his second of the campaign, earning himself a four game ban as a result.
Before signing for Man United, Casemiro hadn't been sent off in his entire career.
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Crystal Palace's loss at Wolves leaves them twelfth in the table and nine points off the relegation spots, as Julen Lopetegui's side moved level with them on 37 points in thirteenth.