Stephen, 40, was acting as quizmaster in the Ant versus Dec segment of the programme when eagle-eyed fans noticed he was trying to cover something up.
Those watching at home spied what looked like a plaster on Stephen’s forehead, and they were fixated on it.
Viewer took to Twitter to flag the cover-up and speculate on what it could be.
“Has anyone else noticed @StephenMulhern‘s plaster above his eyebrow #SaturdayNightTakeaway,” one asked.
While another questioned: “@StephenMulhern What is on your head above your eyebrow? A plaster? #SaturdayNightTakeaway #ITVTakeaway @antanddec @ITV
“What’s with the mahoosive plaster on Steven Mulhern’s forehead #SaturdayNightTakeaway Maybe the plaster should have it’s own #tag emoji,” a fan suggested.
A fourth added: “@StephenMulhern erm. The plaster was all we could look at. What did you do? #saturdaynighttakeaway #AntandDec.”
“Can’t un see that plaster on @StephenMulhern ‘s head @itvtakeaway @antanddec #saturdaynighttakeaway,” another chimed in.
Fans didn’t get to the bottom of the mystery, however, as nothing was said about it on the show.
The pair celebrated the 100th episode of the show which has been running for 15 seasons with an extravaganza of an episode.
Stephen has been a part of the programme for three of those when he joined the team in 2016.
The show wasn’t completely drama-free, however, as star announcer Cuba Gooding Jr. appeared to snub the pair live on-air.
After Ant, 42, and Dec, also 42, enthused over one viewer who claimed to have watched every single one of the 99 episodes of Saturday Night Takeaway, Cuba, 50, held his hands up.
“See, I’ve missed 99 of your episodes,” he teased.
Ant and Dec’s Saturday Night Takeaway continues Saturday at 7pm on ITV.