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.
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)
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.
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: 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.