In TOW the giant poking device when Janice is breathing into a bag cause she's hyper-ventilating, Chandler grabs the bag from her. When they were filming this the bag acctually popped but both actors ignored it and carried on. Janice hid her face in the bag to stop herself from laughing and at first Matthew Perry (Chandler) carries on with a straight face but there's a split second where he nearly loses it and starts to laugh. Watch his face and you'll see. This isn't a lie, Maggie Wheeler who plays Janice noticed this and was later quoted in a Friends book.