In the last UEF mission of Supreme Commander, when the little Monkey Lord brigade shows up on your island.
For those unfamilar with the game, the MonkeyLord is one of the faction's super-weapons, and probably the signature unit of Supreme Commander. It's very large, fairly fast, amphibious, has a lot of armor and sports a variety of weaponry, including the Microwave Laser which would do somewhere on the order of 12,000 damage a second (enough to kill any unit aside from other super units, SCU's, or upgraded Commanders in under a second). It was a very cost effective and threatening Experimental.
http://images3.wikia.nocookie.net/supcom/images/thumb/e/ed/Monkeylord_insitu.jpg/800px-Monkeylord_insitu.jpg
But anyway, the whole mission is defending an island (technically half of it, since the other half is out of view) and things aren't that bad. The mission is mostly fending off large air and sea attacks and launching a sea based attack on an enemy islands, so I ended up with essentially a U shaped defense network of SAMs and Torpedo launchers, with the very north fairly open. Then comes a very large attack lead by a number of MonkeyLords from the north part of my island.
There was no reason to defend that area with anything, let alone ground defenses that could hold off multiple Experimentals. I was just dumbfounded, it was outside of my box of thinking, it wasn't something I was expecting or at all prepared for. Just left me stunned.
They ran though half my island as I proceeded to nuke-spam my own island, since I had virtually no ground assets to actually combat them with. I stopped them before they destroyed my defense objective, but lost most of my economy, most of my defense structures and most of my SCU's that were doubling as builders and resource generators. Disastrous.