Continuity mistake: In the episode "First Kiss," Justin has a girlfriend, Miranda. But in this episode Justin starts dating this new girl from his class, who's actually the same Miranda. This doesn't make any sense. How is she a new student and how could they just meet if they were dating eight episodes ago?
Continuity mistake: In 3-7 "Employee of the Month", Joe Christie says he was with Monk when he got the news of Trudy's death. He said Monk had been laughing, and that he never heard him laugh again. In this episode Monk and Stottlemeyer get called to the scene of a crime, a women's clinic. In this episode there is a flashback to the exact same location years earlier and to Stottlemeyer getting the phone call about Trudy's death. He and Monk were working a case that day and Joe Christie was not present.
Factual error: Throughout the whole show Godber, 'Bunny' Warren, McLaren, the Prison Governor and many of the guards refer to Fletcher as a Cockney. He even identifies himself as a Cockney on a number of occasions. However, in several episodes he refers to his upbringing in Muswell Hill and in this episode he refers to his having been born there - and we see his old stamping ground in that episode when he gets a weekend's compassionate leave. Nobody from Muswell Hill would ever refer to himself as a Cockney - Muswell Hill isn't even in East London!
Factual error: There is not a snowball's chance of Howard becoming an astronaut. He suffers from cardiac arrhythmia, something about which he is completely open. "Transient" or not this is easily detectable by a simple electrocardiogram, which is a fundamental part of the many medical examinations a potential astronaut will have to pass. He wouldn't even be allowed to start basic training. Note also that during his basic training he shows signs of severe stress and trauma which would inevitably trigger a cardiac episode. Even if it didn't the fact that he was falling apart at the seams would have him removed from the programme for his own good.
Continuity mistake: When Phoebe is left alone at the couch, her mom comes up to talk to her. Before her mom shows up, there is a water bottle in front of Phoebe. After Phoebe's mom sits down next to her, the water bottle moves to the other side of the table without anyone touching it.
Plot hole: Lee and Dawn overstay their tourist visa in the US, then fly back to the UK for the Office Christmas party. They then get into a taxi to go out to the airport in order to return to the US. They're kidding themselves. As I know (to my cost) when you overstay a tourist visa, the US Immigration Service finds out when you check in at the airport on departure. They'll be refused reentry, and probably barred from entering the US for up to six years. (I got a three year ban.) Neither Lee nor Dawn could possibly be unaware of this nor think they could beat the system as they would have been arrested when they checked in in Florida on departure (as I was), and they would have remained in small holding cells until the paperwork banning their return had been processed and their passports stamped 'Invalid for U.S. Entry', as mine was. (All this takes about seventy two hours, so they'd have missed the Christmas party anyway.) The mistake is NOT that they were able to leave the USA - that is explained (it's still not possible, but in the context of the show and the suspension of disbelief we are meant to believe it) the mistake is that they are on the way back to the airport to return. That's absurd. They would have been under no illusions as to their banned status re reentry to the USA.
Plot hole: When teenage Prue has run away with the demon, and Phoebe and Piper find her in the park, teenage Prue is 'killed'. Her sisters drive her home to get Leo to heal her, but shouldn't they have just been able to call to Leo to orb to them when Prue was first hurt, and heal her in the park, and save driving back to the Manor and risk Prue dying?
Plot hole: When Zach Young's grandfather died, Zach was said to be the sole heir of the fortune. However, Noah Taylor (the grandfather) had a daughter that appeared several times while Mike Delfino was looking for Deirdre. Where was Kendra Taylor while Zachary Young was spending Noah's fortune on dresses for Gabrielle Solis? Why didn't she claim her share? She simply disappeared.
Continuity mistake: Lily and Marshall mention that they've been using the same toothbrush ever since they've been together, but in the 14th episode of the 1st season, "Zip, Zip, Zip", Marshall and Lily are simultaneously brushing their teeth with a tooth brush each (before Ted and Victoria get home). (00:05:05)