Deliberate mistake: Very often, during Skype or other video chat conversations, the people on the computer screen look left or right when talking to someone who is not directly in front of the screen. Although it certainly helps the viewers understand who's talking to whom (which is why I assume it's a 'deliberate mistake'), it isn't realistic, since it implies that the people on the screen are actually constantly looking away from their computer instead of at their screen or webcam.
Other mistake: In the scene where Leonard threatens to open his production era, we see the Star Trek The Next Generation Geordi LaForge (w/o visor) action figure. The uniform worn by the action figure is only worn by Geordi in Star Trek Generations the movie, not the television series.JetBlazer
Continuity mistake: Bernadette holds her wine glass in her left hand, holding it from the base of the glass; in the next shot she's drinking from her right hand holding the top of the glass and in the immediate next shot she's once more holding the glass from her left hand. (00:14:40)tavares
Continuity mistake: When Howard is proposing to Bernadette she is holding a tray with two glasses on it. The glasses are next to each other, side by side for two shots of her. Then in the third shot the glasses move so one is behind the other. Then in the fourth shot they are side by side again.
Continuity mistake: In nearly every episode, Sheldon demonstrates his OCD when he knocks three times on a door, says a person's name, and repeats that sequence twice, for a total of three iterations, but in this episode he radically departs from that behavior not once but twice. First, wanting to talk to Leonard, he knocks four times, says "Leonard" and repeats the sequence three times for a total of four iterations. Later, wanting to talk to Penny, he knocks four times and says her name, repeating that sequence at least four more times. Given that this was the first season, I assume the writers had not yet settled upon the 3x3 routine as a comic behavior.Matt Wills
Continuity mistake: In this episode Leonard has a snoring problem (a result of a deviated septum, which in all probability he was born with, plus Amy says Sheldon has always complained about Leonard's snoring, and Sheldon says after 5 or 6 years he's gotten use to Leonard's snoring), but in episode 14 of season 3 when Sheldon brings up snoring Penny says, "Leonard doesn't snore." And in episode 8 of season 6, Sheldon lists not snoring as one of Leonard's positive traits.
Other mistake: When Mary is rubbing VapoRub on Sheldon's chest, she makes a comment that last time she saw that there was no hair, to which Sheldon replies "Yeah it filled in last year." In Season 1, Episode 11, Penny rubs VapoRub on Sheldon's chest and a very clear remark about chest hair was made. Season 1 for the show was 4 years ago.Ashish_Agrawal
Continuity mistake: Near the start, when the camera is on Raj, David Underhill is seen in the top left-hand corner of the screen shaking someone's hand, already in the cafeteria. Then, when the camera is on Wolowitz, David is just walking into the door of the cafeteria. David sits down to the right of the man sitting at the table, then shortly afterwards David is now sitting where the other guy was, originally.