It has been hailed as one of the best TV dramas of 2017.
But the latest nail-biting instalment of police anti-corruption series Line of Duty - which was filmed in Belfast - has left many viewers perplexed.
Detectives in Sunday night's fourth episode failed to spot DCI Roz Huntley's badly-wounded left arm.
The gruesome injury was apparently sustained during a violent confrontation at the end of episode one which led to the death of forensics expert Tim Ifield.
But none of the main characters in the BBC series, which follows the fictional AC-12 unit, appeared to notice that Huntley - played by Thandie Newton - was barely able to move her arm. Many of the near six million viewers who tuned in - the highest overnight audience since the series began in 2012 -were a lot more eagle-eyed.
And they were quick to point out what Superintendent Ted Hastings - played by Fermanagh actor Adrian Dunbar - and his AC-12 sidekick had missed.
A series of angry fans took to Twitter to question how Huntley's injured arm had remained unnoticed. One wrote: "Why has no one noticed Roz endlessly touching her wounded arm?"
Another added: "When will they notice she is doing everything with one arm, aren't they detectives?"
A reference to former First Minister Ian Paisley also caused amusement among viewers.
During a heated interrogation scene, Huntley's solicitor pulled up Hastings on his Northern Irish accent and use of cliches, telling him: "Let's dial down the Ian Paisley".
But it is the mysterious case of Huntley's arm which caused most consternation. The full gravity of her wound, somehow gained during the yet-to-be solved death of Ifield, was revealed after Huntley finally relented and went to the doctors.
With two episodes to go, the identity of "balaclava man" - the masked killer who has dominated this year's series - remains a mystery.
Despite the controversy Sunday night's episode is celebrating its highest overnight audience since the series began in 2012. A total of 5.7m viewers tuned in - a 27% share of the audience.