The One With Ross' Step Forward - S8-E11
Other mistake: In the opening scene when Rachel says "I went from store to store sitting on Santa laps" the camera is out of focus.
The One With Phoebe's Rats - S9-E12
Continuity mistake: When Gavin and Rachel are in Rachel's office, in some shots Gavin is wearing a wedding ring. (00:03:43)
The One With the Holiday Armadillo - S7-E10
Revealing mistake: When Ben sits on the sofa to listen to Ross talk about the festival of lights, you can see Monica move Ben to the other side of the sofa despite there being space for her to sit. This is so they sit in the correct order as she would mask Ben to the cameras.
Continuity mistake: When Phoebe goes to visit her mom for the first time, she takes a cookie from the cookie jar and leaves the lid off. But in one shot, the lid is back on. The lid is back off in the following shots. (00:06:36)
The One With The Princess Leia Fantasy - S3-E1
Other mistake: When Phoebe makes fun of Ross' Princess Leia fantasy with the rolls, Rachel tries to stop her; the shots used are the same action, just from different angles. Look at Rachel's hands and the towel in her hand when Phoebe puts the rolls by her ears.
The One In Barbados, part 2 - S9-E24
Continuity mistake: As Mike and Monica are playing ping-ping, two people playing checkers magically materialize behind Monica. One second they're not there and the next second they are. (00:12:25)
The One Where Everybody Finds Out - S5-E14
Continuity mistake: When Monica and Chandler come in to ask Joey about Pheobe knowing about their relationship he throws the Hugsy teddy behind him. In the next shot we see behind the chair and Hugsy is not there.
The One With The Free Porn - S4-E17
Other mistake: 10:50pm boarding and departure, but it is broad daylight outside. (00:09:25)
The One With The Giant Poking Device - S3-E8
Continuity mistake: When Ben says Monica's name she's holding the Minute Maid juice box, but in Ben's closeup the juice box is on the table, though it's back in Monica's hand in the next shot. (00:08:50)
The One Where Monica Sings - S9-E13
Visible crew/equipment: After Joey dips his finger into the hot wax and smears it all over his hands, when Sonya walks out of the back room we can see her tape mark on the floor, which she promptly steps on.
The One Where Monica Sings - S9-E13
Continuity mistake: When Phoebe, Monica and Rachel are talking in the coffee house, a waitress walks by and in the next shot when the camera focuses on Monica, and comes back to Rachel, the waitress walks by again. (00:05:00)
The One With The Embryos - S4-E12
Character mistake: When they are about to start the game, Ross declares that "the first team to answer the most questions wins." This is a nonsensical statement. By that logic, the first team to answer a question correctly would be the winner. It should be either the team that has correctly answered the most questions at the end of the game wins, or the first team to correctly answer an X number of questions.
Suggested correction: While it's a weird way for him to say it, he starts by saying "each team will answer ten questions", so no, the first team to answer a question wouldn't win.
Yes, he qualified that it was a 10-question game, and we as the audience understand going in how the winner is decided. It's just the way he declared how the winner would be decided didn't make sense. It would be like watching an NBA game, and before tipoff, the crew chief tells both teams that the first team to score the most baskets wins. We know what the rules of an NBA game are, but the referee makes a nonsensical statement all the same.
Except there's so many variations of how a team can win a game; best 3 out of 5, win by 2, or a game can even end in a tie. Since this isn't a professional sport where all the players know how a game ends, Ross is basically saying after 10 rounds, if there is a tie, it will go to sudden death with the team with the first correct answer winning, just not in so many words.
The One with the East German Laundry Detergent - S1-E5
Character mistake: Janice enters Central Perk with a large shopping bag and even shows Chandler what she bought but when she leaves a few hours later, she leaves without it.
The One with the East German Laundry Detergent - S1-E5
Revealing mistake: When Ross and Rachel are looking at the pink clothes in the washer, it's clear the clothes are actually dry. (00:17:20)
Factual error: This is a mistake for two of Ross' three marriages. Ross wasn't even actually "married" either time. With Emily, he goes through the ceremony of the marriage and then Emily immediately disappears. You're not actually married until the certificate is signed and sent in, with the signatures of the witnesses. Which was not happening after that disaster of a wedding. Nor were there any witnesses filling out and sending in certificates after the drunk Vegas wedding either.
Suggested correction: The certificate of marriage is signed and sent in by the person performing the ceremony, and he asked if they wanted to continue with the marriage and it was completed. Secondly the marriage to Rachel they discuss how they paid witnesses to the wedding. I believe they mention Elvis?
Suggested correction: At the Vegas wedding, the chapel likely provided witnesses who took care of the paperwork.
The One After the Superbowl, part 2 - S2-E13
Revealing mistake: Rachel grabs a jar of sauce from the shelf and says, "You give me back my sweater or it's handbag marinara." You can tell from the sound it makes when it opens - no pop - that the jar had already been opened before she pulls the lid off.
Factual error: When Bonnie shaves her head, Rachel is supposed to have given her the razors (according to Bonnie.) Nope - the kind of razors needed to do that kind of close shave are a very particular kind, and Rachel isn't going to have them unless she also shaves her own head. It also wouldn't make any sense for Bonnie to have used her own razors; she's not going to carry them around with her unless she's planning to use them.
Suggested correction: Considering Bonnie always used to shave her head, it's not beyond the bounds of possibility that she just kept a razor on her in case she wanted to do it again. She's an impulsive type of person, after all.
The One With The Dollhouse - S3-E20
Other mistake: The dollhouse is only partially burned when Ross takes it into the bathroom to extinguish it. When he shows it to Phoebe it's completely blackened and destroyed. He and Monica wouldn't have just let it burn; they would have extinguished it ASAP, and there's no reason why that wouldn't have been fast if they doused it in the shower.
The One Where Ross Dates a Student - S6-E18
Other mistake: Ross' semester supposedly just ended, and he's handing in his grades. But the episode just before this one was about Valentine's Day, and the one immediately after this one is about spring break, so this episode is happening sometime between mid - February and March/April. It doesn't matter when the show was aired or produced; Valentine's day is still in February. There's no semester that ends at that time of the year, and NYU wasn't on the quarter system then.
Suggested correction: This episode (production code 225567) was produced before episode 17, "The One With Unagi", (production code 225568) and just aired out of order. So this episode happens before Valentine's Day.
The One Where They All Turn Thirty - S7-E14
Plot hole: There's no way Phoebe has lived to 31 without knowing her birth date. She's had "real" jobs (like the massage parlor) and those require social security numbers and accurate birth dates. She didn't go to high school, but she mentions classmates, so she must have gone to elementary/middle school, and those required birth dates even in the '70s and '80s. She's also "lived in Prague," which she couldn't have visited without a passport - which requires a birth certificate.