Continuity mistake: Nessa goes home to Pam's house along with Stacey and Gwen, and in that scene you can see the tattoo on her arm of the Welsh dragon. If you focus enough you can see the dragon only has 3 legs, whereas the Welsh dragon has 4 legs, as does her tattoo usually. Both writer Ruth Jones and director Christine Gernon confirmed this on an outtakes special.
Other mistake: After the wedding when everyone is outside, Smithy says he has forgotten the maps that gave directions to the reception. Bryan says he can use the sat nav in his Picasso and tells everyone to follow him. But he was driven to the church in the Bentley with Stacey, so his car wouldn't have been there.
Factual error: Dave and Nessa's wedding is meant to be on 13th June and at the end supposedly 6 months later Nessa, Gavin and Stacey are sat on the sea wall in beautiful sunshine in summery clothes when it should be middle of December.
Continuity mistake: At the reception, Dawn is talking to Pam and Gwen about the fact that she and Pete's sex-life is dead. Then, Smithy taps on a glass to indicate the speeches beginning, but as the glass is being tapped Smithy is in the shot with his blazer on, talking to Bryn. Then the shot pans over to the other end of the table, and Smithy is suddenly stood up with his blazer off.