In the flashback where Chandler and Monica jump into bed together, both clothed, Chandler starts ripping his pajamas off under the covers. In the second shot of this, when he's removing his bottoms, you can see the strap on Monica's shoulder showing she still has her dress on. The very next shot you can see the strap is gone and she is supposed to be completely naked. Her clothes just vanished.
When Phoebe burbles goodbye to Chandler on speakerphone, her hands switch between shots from clasped in her lap to behind her leaning on the chair.