I've been on a (non BA) flight where, twice, the cabin was dangerously insecure. One memorable flight with Meridiana on an MD83 we had an FA who failed to get herself secured in time for both take off and landing.

Meridiana's MD83s had a curious fold down seat for the FA looking after the rear airstairs emergency exit which has the FA effectively sitting in the aisle - it can be seen here on the left-hand side, stowed. As we left the ground she was still struggling with deploying this seat mechanism and getting herself strapped in. On arrival she hadn't even got back to the seat as we touched down: she was standing in the aisle braced between two seats