The Last Shout: Part 1 - S3-E7
Revealing mistake: Saffy is ironing her boyfriend's shirt and when she has finished he puts it on just before asking her to marry him. When he buttons it up he does so with the buttons and holes misaligned by at least 2 button-holes, and with the collar obviously at an odd angle around his neck.
Audio problem: In the scene where Eddy is in the helicopter and shouting down to Patsy who is on the ground, she says something like, "I'll meet you down there!", but you can tell that line was pre-recorded because her mouth moves completely out of line with the words.