I thought this might be of interest to the discussion.
A few years ago whilst on expedition, a friend of mine was prussiking up a 40m pitch when he noticed his central mallion had completely detached itself from the right-hand loop (if you were looking directly at the person wearing the harness). Luckily, he was a few metres away from a big ledge so he ascended carefully to that and reset the central mallion so it was safe again.
Anyway, we dissected the incident later on that day and came up with the following ideas.
The gear being used was a MTDE sit harness, Petzl Omni (screwgate) and the rest was pretty standard SRT kit with a couple of heavy tacklebags to carry.
1) Firstly, the screwgate Omni can and did undo itself, either by vibration, rubbing against clothing or just forgetting to do up.
Probably best to keep checking regularly, even midway up a pitch if its a long one.
2) Secondly, the tackle bags were attached to the bottom of the Omni. This may have caused the screwgate to open but most importantly, they generally mean the Omni is pulled to one side about 45 degrees by the weight of the bags.
My solution is now to always attach tackle bags to the leg loops, using a little quick draw sling on each leg loop so if one snaps (unlikely), your bags aren't going 40m down a pitch. I've seen people use a piece of cord attached to the loops for the central mallion but I think having weight pulling on that part of the harness is never a great idea as its more stuff to tangle up.
3) Thirdly (and finally) the sit harness was quite slack due to having worn more clothes deeper into the cave. This meant that there was some play in the waist loop of the harness. As the central mallion was at 45 degrees, this meant the loop could push against the unscrewed gate and with the motion of prussiking upwards, this pushed the gate open and then the loop could slide off, causing what I can only imagine would be a large tightening of the sphincter muscle
Solution: Keep your harness tightened!!
Anyway, to relate this to the previous posts, I think this could potentially happen to any central mallion no matter which way the gate does up, so perhaps the lesson to learn overall is to keep checking and ensure the gate is working correctly