Character mistake: In the final show, when Nesa and Smithy are getting married, Smith states his name as Niel Noel Edmund Smith, but that is actually his son's name, a.k.a. Niel the baby.
Episode Six - S1-E6
Continuity mistake: When Gavin and Stacey have their first dance, look carefully and you can see Nessa sat at a table behind them with other people. Seconds later, the camera moves and focuses on Nessa standing at the bar alone.
Trivia: The three main families in the show are named after British serial killers (West, Sutcliffe and Shipman).