Phoebe's Mom had long hair at the end of the previous episode, but now she suddenly has short hair, even though it's a continuation of the scene at her house.
Bonnie tells Ross that she got a bad sunburn on her head. But she had gone swimming at night only once after shaving her hair, so she couldn't have gotten a sunburn.
Chandler's hair goes from short brown to longer and blonde in between the season 3 finale and season 4 premiere at the beach house.
During the last shot of the previous episode, when Ross opens Rachel's bedroom door, the doorway is right next to the far wall of Rachel's room. At the beginning of this episode when we see Ross inside the room, there is enough space between the far wall and the doorway for a dresser.
Urinating on a jellyfish sting would not cure it but would actually aggravate the injury by releasing more venom.
When Phoebe leaves her birth mother's house, her green purse is still visible in the background on the counter. But in the next scene back at the beach house, the green purse is on her shoulder as she walks past Chandler, Joey, and Monica and heads upstairs. - Phoebe comes back in with her hand over her eyes since she said she never wants to see her birth mother again, asking where her purse is. This scene was likely cut short for television broadcasts but explains why she has it coming back to the beach house.
When Phoebe visits Ursula, there is a mismatch in the dialogue. Phoebe's mouth is moving even though her dialogue has been muted to allow for Ursula's response.
Chandler says he can sometimes hear Joey screaming at him late at night like he did at the beach. Joey says that he just does that to freak Chandler out. However, Phoebe, Ross, and Rachel are all in the same clothes as they were at the beach indicating that it is still the same day. Therefore, Joey would not have been able to do the shouting late at night as they have not slept yet and certainly could not have been a few times as was implied.