Narrative:

The main cabin door opened on takeoff roll about 40-60 KTS. There was no annunciator prior to the door fully opening. It was later discovered that the annunciator system was malfunctioning.

Google
 

Original NASA ASRS Text

Title: A BE1900 ON TKOF ROLL AT 40-60 KTS HAD THE MAIN CABIN DOOR OPEN WITH NO DOOR WARNING ANNUNCIATION WHICH LATER PROVED INOP.

Narrative: THE MAIN CABIN DOOR OPENED ON TKOF ROLL ABOUT 40-60 KTS. THERE WAS NO ANNUNCIATOR PRIOR TO THE DOOR FULLY OPENING. IT WAS LATER DISCOVERED THAT THE ANNUNCIATOR SYS WAS MALFUNCTIONING.

Data retrieved from NASA's ASRS site as of July 2007 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.