Ross and Rachel were writings out their vows together for Monica and Chandler's wedding. Then suddenly, a little later, they got drunk and suddenly got naked and had sex and ended up sleeping together. This one-night stand resulted in Rachel becoming pregnant with Ross's baby. This was Ross's's second child and Rachel's first. In the Season 8 finale, Rachel gave birth to her and Ross's daughter, Emma.
In "The One With The Videotape", it was revealed that the one-night stand had been caught on tape.