The Rail Accident Investigation Branch (RAIB) has released its report into an open passenger door on a moving train near Desborough on 10 June 2006.
The full report is available here:

A summary of report is: On Saturday 10 June 2006 an exterior door on a train from St Pancras to Sheffield came open while the train was moving. The door came open on the Class 222 Meridian just north of Kettering, causing the train's brake to apply automatically. However, the driver initially overrode this as indications in his cab were ambiguous, and he was uncertain what had happened. When the driver realised the situation, he made a controlled brake application and brought the train to a stand at Desborough summit, six miles north of Kettering station. The door was then closed and secured, and the train continued to the next station, where it was taken out of service.

There were no injuries or material damage as a result of the incident. However, the state of the door after the train departed from Luton station presented a real and unprotected risk to those on board the train.

The immediate cause of the incident was a malfunction of the door lock switch, which detects the status of the door lock, when the doors were opened at Luton station. This meant the door was both unlocked and had no closing force applied to it from the door motor. There were also nine contributory factors to the incident.