When it was suddenly revealed that when Monica hooked up with Chandler, she had actually been wanting to sleep with Joey.
Firstly, it completely destroys the idea of a charming romance between Monica and Chandler.
Secondly, it completely destroys the idea of a friendship of equals between Joey and Chandler.
Thirdly, quite clearly in the original episode that was not what was intended, and Chandler and Monica had a heart-to-heart in the reception just before.
Fourthly, it was just not very likely, unless the female cast members were constantly trying to sleep with Joey every time they felt a bit down in the dumps.