Some units crash, or nothing happens.- a comprehensive list of the effects on units on cloaking can be found here
http://www.staredit.net/index.php?showtopi...85&st=0#topic50A while ago when Patch 1.12b wasn't out yet, Player 161 can be used for cloaking. But not anymore.
Edit: Such as a hydralisk is really unstable to cloak. However, marines and firebats or bunkers can be cloaked easily via the tutorial triggers.