Rachel, Tag, and Pheobe exit the roof and leave the door open behind them. A moment later, the camera cuts to Joey and the door is seen closed behind him because evidently he grabbed the pipe that had been propping the door open, however Joey remains within the camera shot the whole time, so he could not have reached the pipe to pull it out of the doorway.
Ross appears to be confusing comets and meteors - bright comets can often be visible in the sky for weeks, and their apparent movement is far too slow for the naked eye to discern. They do not "streak" across the sky like shooting stars.
Despite what the fireman says it is actually not against the law to disconnect a smoke detector but should be discouraged in the event of a real fire.
When Ross and Joey find themselves locked on the roof, Ross pulls on the locked door and the entire wall unit shakes.
When Chandler could not sleep, he switches on the bedside lamp. But on the next shot it is off.
After Phoebe throws the smoke detector wrapped in her blanket down the trash chute, a fireman returns it and scolds her for disconnecting it, but why was the fire department even searching the trash bin for her building in the first place? The ONLY possible explanation for this is so the writers could create more jokes.
Notice when Ross and Joey are arguing over the Bapstein King comet, where Ross is trying to explain to Joey how Bapstein was a very well respected astronomer, Joey can be seen subtly mouthing Ross' dialogue.
Despite being the same building, the roof is clearly smaller than the one that appeared in The One with the Ballroom Dancing (1997), where Joey and Mr. Treeger danced on.