The most interesting panel NOT closed story I have heard was on a DHC-7. A friend of mine that was ferrying the aircraft out on PHNL was having problems transferring and using the fuel system on the aircraft and they had to return to point of origin. Turns out that an ex-Hawaiian Air mechanic (they used to operate DHC-7's) walked to the right hand side of the tail area of the aircraft, un-latched the ground refuel control panel cover and reclosed it. There is a microswitch located on the panel that changes the configuration of the fuel system to allow for refueling. When the panel was originally closed the alignment was just slightly off and the switch did not get activated. After that re-closing the panel they were good to go!
Since the crew was just ferrying the aircraft and it had been a while since the type-rated captain flew them regularly it might have been a small over-sight in procedures. I don't know if there was any cockpit indications that would show if the panel was closed/open.