The Wedding - S1-E12
Continuity mistake: In this episode, Lily is in Robin's house with her dogs. In the episode "World's Greatest Couple", Robin reveals that Lily can not stay at her apartment because Lily is allergic to dogs.
Suggested correction: Doesn't mean Lily can't go over and visit for a while. My mother is allergic to cats. She can visit her cat-owning friends for a while, though she would never stay overnight. Pet allergies can be pretty controllable, as long as you're not constantly in contact with the animal.
Continuity mistake: In the previous episode "The Wedding", Ted and Victoria first see each other whilst Ted and Barney are sitting together and both drinking champagne. In this episode, when the same glance is exchanged, a bridesmaid is sitting with Ted and Barney, and Ted is now drinking what appears to be whiskey. (00:01:00)
Continuity mistake: When Ted and Victoria are first talking, the straps on her dress keeping changing position between shots. Sometimes they are starting to slip off her shoulders, sometimes they sit straight, without her every adjusting the dress. (00:02:10)
Continuity mistake: The morning after The Wedding, when Lily is talking about throwing the bouquet, the mug in Marshall's hand changes position, vanishes and then reappears instantly between shots. (00:05:20)
Continuity mistake: When Lily and Robin are talking at the apartment, Lily's necklace keeps moving from on top of her jacket to hanging straight down over her dress.
Continuity mistake: When Robin and Barney meet up for drinks, the amount of ash on the end of Barney's cigar goes up and down frequently between shots. (00:06:10)
Continuity mistake: When Robin leaves to get Battleship, Barney thinks they're going to have sex, whips his belt off, and throws it on the floor. When Robin returns, he's neatly folding his pants and draping them over a chair, and his belt is still threaded through the loops. (00:16:20)
Suggested correction: He could have easily done that himself so it was easier to put back on afterwards.
This correction is too much of a stretch. It would make no sense for him to take off the belt and throw it to the ground in the first place. He was alone at that point, so he wasn't doing it to impress anyone and he's been in this position before.
Continuity mistake: When Barney is pointing out the woman in the blue shirt at the bar, Robin's drink changes from one hand to the other to the bar between shots.
Game Night - S1-E15
Continuity mistake: In the flashback we see Barney get handed a crumpled flyer. When we see the close up of the flier it is no longer crumpled up. (00:14:00)
Game Night - S1-E15
Continuity mistake: When Barney holds out his phone to show the video of himself with Shannon, it repeatedly switches between his left and right hands. (00:19:30)
Life Among the Gorillas - S1-E17
Continuity mistake: At the karaoke bar, just before Lily storms off, the song book changes position on the table between shots.
Life Among the Gorillas - S1-E17
Continuity mistake: When Lily and Marshall are talking at the apartment, just after Marshall gets off the phone to his friend from work, Lily's necklace changes position between shots. Sometimes it hangs straight down and sometimes it is to one side.
Life Among the Gorillas - S1-E17
Continuity mistake: At the karaoke bar, when Marshall is deciding which song to sing, his beer bottle keeps switching hands between shots.
Nothing Good Happens After 2 A.M. - S1-E18
Continuity mistake: When Robyn is leaving at the start, the dogs change place instantly. Note the Dalmatian. (00:01:30)
Nothing Good Happens After 2 A.M. - S1-E18
Continuity mistake: In the classroom scene, the girl with the black dress with green sleeves keeps changing position between shots. Her arms are sometimes folded, sometimes flat on the table, and sometimes she is leaning on her elbows. (00:01:40)
Nothing Good Happens After 2 A.M. - S1-E18
Continuity mistake: In the first scene the karaoke bar, Barney's drink moves between shots from in his hand at his mouth to set on the table. (00:07:40)
Nothing Good Happens After 2 A.M. - S1-E18
Continuity mistake: In the bar when discussing Ted going over to see Robin, the beer bottles in front of Marshall keep moving with no-one touching them. (00:07:50)
Nothing Good Happens After 2 A.M. - S1-E18
Continuity mistake: When Ted arrives at Robin's apartment, he gets out of the taxi. Camera cuts to outside, and he opens the door and gets out again. (00:09:50)
Nothing Good Happens After 2 A.M. - S1-E18
Continuity mistake: When Ted arrives at Robin's apartment the amount of greenery sticking out from his shopping bag keeps changing between shots. (00:10:00)