Ok this is going to be pretty vague because I have no idea how it happened and I haven't tried to re-inact it yet (it just happened like... 5 minutes ago).
Well I was playing some map called "Extreme Nexus Destroyers" and I got instantly acquanted with my ass. So when my Nexus died, I basically had to cheer on my team mate until he died (which was soon after). But I still had some hyrdalisks left over. So I decided to stack them for shizs and giggles.
I only had 3, so as everyone knows I burrowed one and hotkeyed it #1. Then the other 2 I pushed u, ctrl+1, 1, u and it worked BUT... one of them was burrowed like normal (stacked) but the other was up and CLOAKED!! The only units I had left in the game were 3 hydralisks so there is
no possible reason that hydralisk should have cloaked. You can't even get arbiters in that game. I didn't get a chance to see if it was indeed stacked becuase the other hydralisk was killed soon after.
Here's a screenshot:
That's my hydralisk attacking a bunch enemies becuase they can't see it.
The ONLY thing that I could think that was abnormal when I stacked my hydras is RIGHT when I did the stack command, my teammates ultralisk ran RIGHT next to (almost on top of) it and it didn't quite get a chance to go down into its hole. If a stacked hydralisk attacks an enemy it will pop up out of its hole so maybe this has smoething to do with it.
W.T.F. MATE?!
ADDITION:
Oh yeah... and I WOULD have a replay for you... but when I tried to click "save replay" starcraft froze................. which added even more confusion.
If this is something that can be re-inacted..... that means cloaking for zerg w/out arbiters... OMFG