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)
Revealing mistake: While showing the clip of Marshall streaking the first time, you can see thumbnails for videos of him streaking in New York. Those videos were created later that day and could not already be posted. One also shows a woman in yellow and appears to be a retake of the scene. (00:06:15)
Continuity mistake: In Ted's flashback where Barney's mysterious saboteur is talking to Lily, the girl has straight blonde hair. In episode 14, "The Bracket" where his saboteur was first introduced and she originally spoke to Lily, she had curly blonde hair in the same sequence.
Continuity mistake: Ted says that Marshall and he only ate jerky and drank Tantrum on their road trips. In 2-17 "Arrivederci, Fiero" however it's established that Marshall had a rule forbidding any kind of food or drink, even groceries, in his Fiero. (00:01:15)
Character mistake: Marshall's whole career focus is saving the environment, so it's a poorly written character choice that he buys plastic glasses to avoid a dish-washing feud, boasting that they can just throw them away. (00:20:10)
Other mistake: Robin says she burned the dinner because she mistook the temperature scale on the oven for Celsius. However a given temperature above -40 is always higher in Fahrenheit, so by assuming the oven to be labeled in Celsius she would have undercooked the food. For example: If the directions were to cook at 400°F, she would set it around 200°F (thinking the oven was Celsius.) If the instructions were to cook at 200°C, she would still set the oven to 200°F, undercooking the food in both cases.
Continuity mistake: After the gang gets to the bar around the end of the episode, Ted starts his speech about how they all need to move on. As he moves over to the booth, he tells the people sitting there to get out. You can see that the girl takes her phone with her, and there is no other phone anywhere on the table. A few seconds later, she comes back saying that she left her phone, and now the phone is visible on the table. (00:19:25)
Continuity mistake: In this episode we learn that Barney doesn't know how to drive. But throughout the whole series there are reference that he actually does, such as: In 'Come On' (22nd episode 1st season) : Penelope says to Barney: "We had sex twice in your car." In 'Moving Day' (18th episode 2nd season) : Barney drives away with Ted's rental van. In 'The Bracket' (14th episode 3rd season) : Barney stole Holly's SUV as they were camping.