season 6 episode 2 (the one where ross hugs rachel)
season 6 episode 4(the one where joey loses his insurance)
season 6 episode 5(the one with joey's porsche)
After Ross confesses to Phoebe that he didn't get the annulment, Phoebe forces him to avoid plans he had with Rachel to explain. Ross says he didn't because he doesn't want to have three failed marriages at such a young age. Later, Phoebe tries to prove to Ross that it isn't that big of problem: they tell the whole story to a group of girls, all of whom don't care too much about his three divorces but still would not date him. Each gives a different reason as to why: one of them insists she wouldn't date him while he's still married, another only wants to date troublesome men, and the last says that she thinks he's still in love with Rachel (which Phoebe latches onto as the actual explanation). Ross insists that isn't true and decides to get the annulment, but it turns out that won't be as easy as he'd hoped because they both have to appear before a judge in order to have the marriage annulled. However, Phoebe still does not get this and despite Ross's attempt at an explanation continues to believe he still loves Rachel.
Moments later, Ross walks in, wanting to tell Rachel about the marriage, but she's really upset about moving out, and Ross instead hugs Rachel to comfort her. However, as he hugs her, he ends up accidentally smelling her hair.
While in Ross' apartment, Rachel gets a call from his lawyer who tells her that since he hasn't heard back about the annulment he's assumed they're giving the marriage a shot. Of course, the fact they are still married is news to Rachel. At the University, Ross tries to phase out the accent but everyone notices. He drops the accent completely, confesses the truth and asks for another chance just before Rachel storms in to scream at him over the fact he never got the annulment (causing the accent to quickly return).

Rachel has found out Ross didn't get their Vegas wedding annulled and are still married, and naturally she is furious with him. Rachel doesn't know where to live until Phoebe offers her a room. Rachel decides to take care of the annulment herself because she doesn't trust Ross anymore. Rachel and Ross go before the judge but Rachel filled in false reasons for their annulment. When the judge finds out this and the fact the two were previously involved, she refuses to annul the marriage forcing them to get a divorce instead. Ross and Rachel are furious and Rachel threatens the judge, but when asked if they would want to spend the night in prison, the two leaves immediately. Ross and Rachel sign the divorce papers and Rachel confesses that it was her idea to get married. They share a moment of truth that they both thought if they ever got married it would last.