Donna says it's been three weeks since her attack, then later Cindy tells Jim it took Valerie three weeks to leave the hotel and move back into their house. Valerie was living in the hotel long before Donna was attacked, so it couldn't be three weeks since that happened and three weeks that Valerie was living in the hotel.
When Kelly tells the gang that Mel is moving back in with her mother, she says "How can Mel take Jackie back?" when she obviously means "How can Jackie take Mel back?!", since it was Mel who cheated.