My main two crazy moments were in Rome Total War.
1) I was playing as the Romans, and I had a small settlement somewhat isolated from the rest of my empire but sharing a border with the Gauls. I didn't have the buildings necessary to train anything more advanced than Town Watch, but I had two units of hastati (light infantry) and a General unit from when I conquered the province. All up I had 300 men, and the Gauls invaded with a force of 900, comprised mostly of warbands and a few swordsman units (both light infantry).
I used my town watch units to tie up units while I flanked them with the general's bodyguard unit, while I left my units of hastati to engage other units head on.
Somehow, at the end of the battle, I'd killed the enemy general, inflicted 500 casualties, taken around 100, and routed the rest of the army.
2) This one was more recent. I was playing as Greece, and I was building up a force in Sparta in order to attack Macedon and then capture Athens. However, it looked liked Macedon was gearing up for an invasion themselves (we were neutral at this point). They had two armies in the province, one with 400 men, one with 600 men, and then another army garrisoned in the city with 500 men. Stupidly, I forgot that if I attacked one of these armies, the other two would arrive shortly thereafter to reinforce them. I sent up a paltry force of 500 men, consisting of hoplites, armoured hoplites and a general unit), and attacked the army of 400.
As soon as the battle began, I realised my mistake.
Thankfully, at first I only had to deal with the force of 400, and it was just spearmen vs spearmen, plus a little cavalry flanking. Took minimal casualties (lost maybe 50 men), destroyed around half of their army and the rest routed (however, later they rallied and joined the other armies in their attack).
Once they were dealt with, the two reinforcing armies were coming at me from either side, so I set up my men in an outwards facing circle with the general unit off to the side. The rest was just a maelstrom of spearmen vs spearmen, at some point during which I was forced to sacrifice the general unit.
In the conclusion, I had around 200 men remaining, had inflicted around 600 casualties and routed all three armies.
After this, I promptly withdrew my army.
EDIT: Speaking of maelstrom, I just noticed how many times I used the word 'unit' in this post!