Hi Ira,
The problem often comes from the pilot opening the door to cool the 
cockpit or demist the windscreen at the last minute (possibly after 
having performed the pre-takeoff check)  
and failing to re-lock it properly. 
My suggestion:
1.  Check doors locked (both pins engaged) during your pre-takeoff check 
(I assume everybody does this!)
2; Never open a door after having performed  your pre-takeoff check, 
unless absolutely necessary (in that case double check door properly 
locked)
A few decades ago I experienced a side window popping open during 
takeoff in a Cessna 150. The noise and the wind were frightening! The 
key here is to fly the airplane first and deal with the door or window 
second.  
Best regards
Remi Guerner
F-PGKL
<<<<These episodes,the NTSB says were all apparently due to improper
shoot bolt deployment into the rear socket and failure to check
by crew.>>>>>>>
 
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