Continuity mistake: During the first tape Hannah says that there are 13 tapes as there are 13 sides to every story, however we later find out that she only planned and recorded 12 tapes that night, and the 13th (Mr Porter's) was unplanned and recorded later on, so she would not have included it with the others when she mentioned how many tapes she was going to make in the beginning.
Other mistake: In the original version, a portrait of Bryce's family can be seen. Not only are the actors playing his parents completely different from the later episodes (which isn't necessarily an error, because actors for characters change in shows all the time), but Bryce is also shown to have a sister in the portrait. In later episodes, it is established that Bryce is an only child, so he doesn't have a sister. The portrait of his family is altered on the current streaming version on Netflix to show the new actors for Bryce's parents and remove the sister, but the DVD still has the original version.
Continuity mistake: When we see Hannah in the room (when Bryce and Justin are talking about beer pong and stuff before Bryce walks in), the door stays shut the whole time. But when the camera shows Bryce's side, the door opens slightly, but Justin closes it.
Continuity mistake: When Hannah makes the list with names involving her past events, they differ substantially between the close-up when they're written and the final wide angle once the paper is finished. Check Alexander: first an irregular oval with a straight side, then a perfect oval.
Visible crew/equipment: As Hannah goes to take the cassettes we can see the shadow of the moving camera. (00:04:25)
Continuity mistake: At the end of the episode, the scenery outside Tony's window changes. We first see a body of water and grass, which then changes to a row of houses, grass, and a smaller body of water in the big overhead shot.
Continuity mistake: Mr. Porter and Principal Bolan are having a conversation outside the building when two kids walk by as the shot is focused on Mr. Porter. When the shot changes to Bolan, the same two kids are walking by again.
Other mistake: In the hall during the Spring Fling Dance, Bryce reminds Justin of the night before Freshman Year when they stayed up all night talking about being kings of the school. This doesn't make sense considering Justin is then a Junior while Bryce is a Senior. Bryce does however end up in the same year as Justin when he transfer to Hillcrest for Spring Semester and goes in as a Junior. (00:52:48)
Continuity mistake: When Tyler arrives at the school with the gun, the cuts and scars on his face from hours before are gone.
Visible crew/equipment: When the camera speeds around Clay and his friends as 'The Night We Met' by Lord Huron plays, a lady in a white shirt is seen patting people on the back, likely making sure they are in the right position for the shot.
Factual error: In the disturbing assault scene, after having his head smashed into a mirror and banged against the sink multiple times, there is no way Tyler would be conscious for the rest of the scene.
Plot hole: When Bryce's body is found, there is a gaping hole in the back of his head, leading to speculation that he was shot. It is later determined that the hole was from blunt force trauma. However, when we actually see his death, he is simply pushed off the pier and ends up drowning. It wouldn't have been caused by his fight with Zach, as he would have been dead long before he could have been pushed into the water. And if the wound was somehow caused by aquatic predation, the forensics experts would know the difference between that and blunt force trauma.
Always Waiting for the Next Bad News - S3-E9
Continuity mistake: Jessica is wearing a massive earring, but when the camera is behind her, she isn't. (00:23:38)
Graduation - S4-E10
Continuity mistake: In the scene before the last when Clay is getting out from his house with 2 bags and goes to put them in Tony's car, he has a bag on his back but when he closes the trunk the bag disappears. (00:33:00 - 00:34:00)
