When Monica and Rachel are arguing in the apartment, Monica threatens to shred Rachel's sweater. In between shots, the length of the thread goes from short to long to short again before she even pulls on it.
When Chandler is taking his shoes and clothes off in the bathroom with Susie, one of his shoe is left between his cubicle and the one next to it. This shoe then disappears and Susie collects both shoes outside the cubicle. However the shoe reappears in the last shots of this scene when Susie is leaving with Chandler's clothes.
When Chandler is leaving the restaurant wearing nothing but pink panties and carrying the door in front of him, you can briefly see that he is actually wearing a pair of gray shorts.
Correction: He does indeed appear to be wearing a pair of short gray shorts, however, the last line before Ross and Joey returned to the table, was Chandler pledging 50 dollars for Ross to give him his underpants. Then both Joey and Ross walk to their table a bit conspicuously. It seems that Ross was wearing gray boxers and gave them to Chandler.
When Rachel and Monica are fighting the 2nd time you can see the tape markers on the ground in the kitchen.