This is why I put those plastic hard covers with a rubberized/matte coating on my laptops. While Ive had them protect a MacBook from a moderate fall, the texture makes them non-slippery and super easy to grip.
Yea I had walked up some stairs to the second story of an aircraft hangar and was leaning against the rail and it just slid right out of it's sleeve which I obviously thought was zipped close. Fell and landed pretty flat right on the concrete.
Ouch! I'll bet you'll check the case is closed in the future....
At least you had a case (even if it didn't help you in this instance). MacBooks are quite slippery and I wouldn't walk with mine for any distance over a hard surface without a case.