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)
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.
The Wedding - S1-E12
Continuity mistake: In the first scene at the bar, as Ted heads off to speak to Robin, Marshall's beer goes from at his mouth to on the table instantly between shots. (00:02:15)
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)
Other mistake: In the credits, Jeremy Gabriel (Robin's new 'boyfriend') is shown as "Mark" but it is "Mike" throughout the episode.
Continuity mistake: Robin says that she doesn't like team sports, and so she only played tennis in high school. However, throughout the show, Robin often talks about playing hockey during school, which is a team sport.
Sweet Taste of Liberty - S1-E3
Continuity mistake: After Lily takes off her ring in the bar, she is seen in a later shot with the ring on her left hand.
The Pineapple Incident - S1-E10
Continuity mistake: When Lily says "No dibs" and touches her nose, she does it with her right arm/hand, but in the next shot, her left arm/hand is on it. (00:05:55)
Nothing Good Happens After 2 A.M. - S1-E18
Continuity mistake: When we first see Korean Elvis, Marshall has his hand on his beer. The camera cuts to Marshall and his hand is no longer on his beer. (00:12:00)
Nothing Good Happens After 2 A.M. - S1-E18
Continuity mistake: When Ted walks up the steps to Robin's apartment his bag is all creased and folded. When he arrives at her apartment it has no creases. (00:10:30)
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)
The Pineapple Incident - S1-E10
Continuity mistake: When Ted is explaining to Robyn about Trudy he points at the door. Camera cuts and his arm has been lowered. (00:20:20)
Continuity mistake: When explaining why he has 2 costumes every year, the way Barney holds his drink changes instantly. (00:12:00)
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)
Continuity mistake: At the third party when Lily sits Ted down to talk to him, Marshall stands behind Ted with a beer bottle in his hand. The position of Marshall's hands on Ted's back changes between shots. (00:14:40)
Sweet Taste of Liberty - S1-E3
Continuity mistake: When Marshall is on the phone to Barney, the green highlighter jumps back and forth from on the desk to on top of the open book.
Continuity mistake: After Ted's Saturday night party (second of the three), when Ted, Lily and Marshall are sitting on the couch, numerous things change or suddenly appear behind them. Clothing is instantly on the back of the yellow chair and black chair, a wood chair appears, furniture moves, etc.
Chosen answer: She was finally shown at the very end of the last episode of season 8, continuing into season 9. The actor's name is Cristin Milioti: http://www.imdb.com/name/nm2129662/.
Knever