A passenger aboard Asiana Airlines flight OZ8124 opened an emergency exit door just before landing in Daegu. The aircraft was able to land safely and the passenger was taken into custody by local police. Some passengers received medical attention though no serious injuries have been reported.
Asiana Airlines flight OZ8124
Flight 8124 departed Jeju at 11:58 local time (02:58 UTC) for 42 minute flight to Daegu. According to the BBC, at 250 metres (820 feet) and approximately 1 minute before landing at 12:37 local time, a passenger opened the left side, behind the wing exit door (Door L3). Based on footage of the aircraft in Daegu, the door’s escape slide discharged as normal.

Isn’t it impossible to open a door in flight?
For 99% of the flight, it is absolutely impossible to open an aircraft door. Aircraft cabin doors act as plugs. As the pressure builds inside the aircraft literally tons of pressure push on the door to keep it secured in place.
However, as the aircraft descends to land, the pressure inside the aircraft begins to equalize with the pressure outside the aircraft and at a certain very low altitude the door can be opened. In this case it was about 1 minute prior to the aircraft touching down.
Aircraft info
Asiana Airlines flights 8124 was operated by an Airbus A321-231 registered HL8256. It was delivered new to Asiana in May 2012.