Discovered this totally by accident, totally unable to explain it or predict its appearance so far.
Buying a battlesuit while standing on the pipe near the human base on arachnid2 drops the player through the pipe. I'll attach a demo in a moment, but it's pretty easy to reproduce.
For what it's worth, this bug has been around for a while (discovered on mgdev, based on svn1055) and happens on svn1081 with or without attachment 1724[details] for bug 3595
Created attachment 1725[details]
battlesuit falling through pipe, svn1081
Here's a demo of the bug, taken with svn1081 with attachment 1724[details] applied (although I'm pretty sure the latter point is irrelevant).
It's impossible to open the armoury menu once you are down there, but console commands/binds still work if you build it in the right place.
Created attachment 1811 [details] this seems to do the job It seems that the pmove took over the origin. (One minor tweak to newOrigin[2].)