Audio problem: Any scenes outside Central Perk always sound as if they're on a set (which let's be honest, they are) - the sound is often muted and footsteps on the pavement have a very slight echo to them.
The One Where Monica Gets a Roommate - S1-E1
Audio problem: The camera cuts briefly to Chandler as the friends are discussing Monica's love life. During this very brief time, you can see Phoebe's mouth moving but there is no audio of her voice, and Monica's voice is heard talking in the background instead. (00:01:10)
The One Where Monica Gets a Roommate - S1-E1
Audio problem: When Phoebe says "I just don't want her to go through what I went through with Carl" the camera cuts to Chandler's reaction. When you look at the bottom Phoebe's mouth is moving even though she's not speaking. (00:01:39)
The One with the Thumb - S1-E3
Audio problem: When Monica is first seen in the kitchen complaining about taking a guy home, the other chef is cutting some items up. However, the audio of the knife clicking as it hits the board is overdubbed. (00:08:00)
The One with the Blackout - S1-E7
Audio problem: When Phoebe is counting down to the last candle burning out you can see Joey blowing out the candle, but the sound of his blow is only heard a second later after he sits back. (00:22:50)
The One with the Dozen Lasagnas - S1-E12
Audio problem: The very last line of the episode (Monica saying "and I'm gone") is an overlay. (00:22:40)
The One With the Baby on the Bus - S2-E6
Audio problem: When the lady on the street (played by Lea Thompson) is talking to Joey and Chandler, her mouth and her voice are not in sync.
The One With The Lesbian Wedding - S2-E11
Audio problem: During the lesbian wedding, whenever we see a shot of Carol and Susan while the minister is talking, her mouth is not moving. (00:18:35)
The One After the Superbowl, part 1 - S2-E12
Audio problem: When Phoebe is showing the children her guitar in the library, her voice comes on saying, "I know," when the children are amazed by her guitar. Look carefully - her lips are not moving when she says this.
The One With Barry & Mindy's Wedding - S2-E24
Audio problem: When Rachel asks Barry and Mindy why people keep telling her to 'get well soon' and Barry says his parents told everyone she has syphilis, and in the next shot it shows Rachel trying to take this information in, Barry seems to still be talking, but no sound is coming out of his mouth. (00:09:25)
The One With The Princess Leia Fantasy - S3-E1
Audio problem: When Monica answers the door and sees her father, she says "Hey Dad, what are you doing here?" but her lips barely move at all.
Audio problem: In the scene where Malcolm is talking to Phoebe in the subway, you can hear him but his lips aren't moving.
The One With The Tiny T-Shirt - S3-E19
Audio problem: At the start when Joey comes into Monica's apartment and does his little dance number, watch his mouth on his way out; it's totally out of sync with what he is saying. (00:03:15)
The One With The Tiny T-Shirt - S3-E19
Audio problem: When Joey chants "Oh Mommy, Oh Daddy, I am a big ol' baddie" his voice doesn't match his lip movements.
The One With a Chick. And a Duck. - S3-E21
Audio problem: At the end of the program, Joey asks Chandler if a chick can swim. They put the chick in the bath, but it's still able to chirp even though it is supposed to be under water. (00:21:20)
The One With The Jellyfish - S4-E1
Audio problem: When Phoebe is knocking on her sister Ursula's door to tell her about finding her birth mom, the third knock is heard a split second before you see Phoebe's hand action. (00:12:15)
The One With Joey's New Girlfriend - S4-E5
Audio problem: In the sequence where Chandler chases Kathy down the street, there is a moment, shortly after he walks into the man with the hotdog, where he shouts her name, but his lips don't move at all. (00:11:25)
The One With The Embryos - S4-E12
Audio problem: When Rachel is banging on the door at the start, the sound does not match the actions. (00:01:00)
The One With Rachel's Crush - S4-E13
Audio problem: In the scene where Rachel walks into the guy's new apartment and they start talking about Joshua, it cuts to a shot of her in profile as she says "It's just so frustrating". Her mouth doesn't move as she says the words. (00:09:40)
The One With Ross's Wedding, part 2 - S4-E24
Audio problem: When the wedding officiant says "take thee, Ross" his lips don't move. (00:20:35)