This is in Season Five,"The One With Ross's Sandwich.

Monica and Chandler have a secret affair,which only Joey knows about,and Joey covers for Monica and Chandler when strange situations happen.

Anyway,Joey is coming home with a date.Inside,Monica and Chandler are getting ready to do romantic things on a blanket with pillows and candles around.They hear Joey coming home and run off into Chandler's room(they were in the living room).When Courtney Cox(Monica)got up quickly she accidently kicked a few pillows out of place,next to a candle.

In the next shot,Joey enters and you see the whole blanket,and the pillows are back into perfect place.

Later,in the next same shot,the pillows are messed up in the same way Monica did to them.The whole time,no one was there to move them!