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).
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.
Continuity mistake: In season 5 episode 8, Marshall and Ted are playing on an Xbox 360 Console. If you look carefully, you see that Ted's Xbox controller has no batteries in it, yet he is playing the game with Marshall, and then a few scenes change and so do the batteries. Scene 1, 12 minutes and 23 seconds: Marshall's remotes has a battery in while Ted's is out. Scene 2, 12m 40s: Marshall's Xbox control has no battery in. Scene 3, 12m 51s: Marshall's battery is back in the controller. Scene 4, 12m 58s: Marshall's battery is out again. Scene 5, 13m 44s: Marshall's battery is back in.
Continuity mistake: In this episode we get to see when Ted met Marshall. But in Season 2, episode 1 Ted says to Marshall, "And there was my roommate, lying on the bed with his feet up on the wall. He had headphones on, big smile on his face. That guy was you." In this episode we get to see it was different.
Continuity mistake: In Ted's flashback where Barney's mysterious saboteur is talking to Lilly, the girl has straight blonde hair. In episode 14, "The Bracket" where his saboteur was first introduced and she originally spoke to Lilly, she had curly blonde hair in the same sequence.
Continuity mistake: When future Ted is counting the times Marshall slapped Barney, the first one is shown as the one at the bar, but this was before Barney decided 5 for eternity. The one that counts as the first is the one he gets after they watch Robin Sparkles' Video.
Character mistake: Cindy's claim that there is a policy against faculty and students dating is right, but according to the website of Columbia university it doesn't apply to a relationship between Ted and her. Excerpt from the policy on consensual romantic and sexual relationships between Faculty and Students of Columbia University: "It is the policy of the University that no faculty member shall have a consensual romantic or sexual relationship with a student over whom he or she exercises academic or professional authority."