Boeing 737 taxiing out in the UK. Old lady in front row tells cabin crew she can see out through a gap round L1 door. Ignored by cabin crew. Aircraft launches and fails to pressurise. Air turnback.
Similar thing happened on a BA 707 out of Montreal once. One of the rear doors was very obviously not shut properly after galley loading. Cabin crew sat next to it and just let it all happen. Of course once airborne the screaming airflow noise finally prompted the clowns to tell someone on the flightdeck. After levelling off at 10000 the flight engineer (remember them?) actually opened the door against the airflow and shut it properly. No turnback required.