In TOW Rachel's Sister (Season 6), Monica is unwell with a bad cold, looking and sounding very unwell, and towards the end of the episode, she and Chandler go into their bedroom for sex. The episode ended with Rachel looking into Ross' flat to witness Jill closing the curtains.
The opening scene of the following episode, TOW Chandler Can't Cry, is set directly after TOW Rachel's sister with Rachel still looking into Ross' flat, however when Monica and Chandler emerge from their room, Monica has miraculously recovered from the cold she had 5 minutes ago! It's as if she recovered from the coughing and sneezing within after an hour!