Quite a hard one, i will try to explain, sorry for the long text:
This happened on the current beta and on the update before that one.
Some alien units are bugged if you switch to any commander. You are stuck inside the unit, while having the commander HUD. However, this alone isn't really bad, you can just press H and get in any team as player. But if you stay in it, as soon as you attack with that unit, the whole server starts lagging, or in other words, it hardly responds to anything. The unit keeps attacking, even if you do nothing after the first attack.
Sometimes the bug goes away by itself, but only after you managed to get into another unit, sometimes the round needs to end manually, and one time the whole game crashed for the Host.
This does NOT happen with crabs, horned crabs and wasps, but with every other alien unit.
I've first encountered it on the PB server, then recreated the bug on my own server and on a one hosted by rubue, both ran without mods (i did not mess up other players games btw).
This bug seem to appear on every map and in every strategy game mode (Tested on Riftbasin and whispering plains, SOLvsAlien, CentvsAlien, HVHVA).
It does NOT appear when the Host tries it, only clients can cause it (Tested with rubue as host, rubue couldn't recreate it, i could and the game crashed for rubue).
Also, i've noticed that if you spawn an alien unit on SOL and try to create the bug, it doesn't work, but i only tried that once. Seems to only apply to alien units in the alien team.
Recording and client game log are from a Test on the PB server on beta.
Hope i got everything right, tell me if you need more details.