I've seen some high-security screws that look like perfect circles but are actually veeeeery slightly oblong, so slightly that it's hard to tell. The tool that opens them is also a very slight oblong. The screws in a cable box I once had were this way.
There's a trick to removing them, if you don't mind not getting the screw or the tool back. Find a tool that fits in the circle, like a small pentalobe or hex driver tool. Put a tiny drop of super glue on the end of the tool, being very careful not to let it spill down the side of the tool (you don't want it getting on the threads of the screw). Press it into the screw, hold it for ten seconds, then turn the tool and remove the screw.
If you absolutely must have the screw back, heat it up to melt the Crazy Glue.
