Episode One - S2-E1
Continuity mistake: When Gavin and Stacey arrive at Mick and Pam's house in the Saab, the car stays on the drive. A few minutes later, Bryn appears in his Citroen into the same place on the drive and the Saab has moved to the space in front of the house.
Episode Two - S2-E2
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.
Episode Four - S2-E4
Continuity mistake: As Mick pulls into the driveway his Saab reg is P650 OHK. Later in the same episode it's changed to P361 NVM on several occasions. (00:03:00 - 00:09:00)
Continuity mistake: The scene when Edna comes in, Dave has a plate of food in one hand. Dave says "All right Edna, how's it going, all right?" It cuts to Edna sitting down and you see Dave with his hands suddenly in his pockets.