Originally Posted by Z50Pilot
...but I'm willing to bet that there is a lot of user error involved.
That depends on situations. My own experience is that the sidecases came off due to me not locking properly - thus user error. Even further in the past, while touring PEI on my K100RS, I was chased down by a friendly local (these nice Canadians!) - the sidecase opened up and all my crap fell out. Still user error, indeed.
The GS Vario top case is a different story. The plastic will flex and allow the case to move a wee bit forward under impact. This allows it to slip out from under the latch. BMW corrected that and offers a retrofit for older cases: a pin that prevents it from moving.
They also offer a brute-force solution: the enduro kit is an extra bolt that screws the case down. Obviously, you cannot remove it anymore just by opening the latch. I decided against that inconvenience, I only applied the retrofit and it works for me so far.