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 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: 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.
Other mistake: When Ted and Robin borrow Marshall's Fiero and are sitting in the car talking as Ted drives, you can hear the beat of a song playing in the car that is not the Proclaimers' I'm Gonna Be (500 Miles), which in many future episodes is referenced as the only song that can be played in that car due to the tape being stuck in the tape player.