When Ross and Rachel are talking in the coffee shop, the cups sitting on the counter change positions several times between shots.
Right at the beginning, when the pigeon flies into Monica's kitchen and Rachel catches it with a pot, a wire (probably to avoid the pigeon flying out) is clearly seen both on the table and then coming out from below the lid.
In the beginning of the episode when the pigeon flies into the apartment and is on the table, for a second, the sound of a rooster can be faintly heard in the background.
When Ross swings on the light pole, it shakes.
When Monica tells the gang that Chandler has a third nipple, it takes everyone except Joey by surprise. However, Ross and Chandler were roommates in college, which means they slept in the same bedroom for at least two years. It's hard to believe that Ross never has seen Chandler's "nubbin", especially since Joey, who's never shared a bedroom with Chandler, has seen it.
When Phoebe admits to eating a cheeseburger, the boom mic is reflected in the window behind her, moving horizontally back and forth multiple times.