Classic/Antique Car Repair: rolling in gear, wheel chocks, car ramps

Question
I already came to you for help once on my 1969 c10, but I was working on it yesterday, and i drove it onto a set of car ramps, shut it off and left it in first gear, as i was going to get wheel chocks for the rear, my truck rolled off the ramps, and back into my garage. I lost a mirror and destroyed my driver side door. Why did my truck roll? I have never heard of chevy having transmission issues allowing it to roll while in first. My truck has a 4.1L L6 with a 3spd on the column, any advice would be helpful  

Answer
Shane;
 Wow that sucks, but it can happen , you might have been a half a tooth  in gear or the clutch or pressure plate could be weak. it does happen but not that much thats why the emergency brake is there.do not feel too bad you are not the first and definitely not the last, I had a friend have a con. edison  bucket truck roll back into his foot , steel toes he was lucky