This is an interesting thing that I observed in 2012.

I had noticed on the Internet that someone suggested that you use wheel chocks (look them up on Google)  to support the front wheels of your car when jacking up the back.  The article said that the emergency brake locks the BACK wheels, not the front. The PARK gear, for that matter, locks the back wheels also.  So, if you  prop up the back of the car on jack stands, and if the car is at all unstable, it could just roll backwards.
I had raised my car on RAMPS in the front, and then jacked up the back and put jack stands under the back axles.  I knew to be careful when lifting both back axles at once, since the front wheels were not blocked with chocks, but WERE resting in the cradle, or low spot, that you get when the front is all the way up on the ramps.
What happened was, as I jacked up the back "pumpkin" or rear end, the jack tended to pull the car back a little.  By the time I was finished, the car front no longer rested in the low point of the ramp, but had snuck backward so that the front wheels were right on the edge of the top of the ramp.

I went back to the font, lifted each front corner in turn and pushed the ramp backward so that the front was again all the way on the ramp top.  I did this TWICE, since after the first time I went back to the rear and raised the jack stands another couple of inches.  Again, the front wheels snuck back on the ramp.  The photo below, shows the amount that I had to move the ramps backwards to make the front wheels rest in the correct spot.