My biggest blunder was attempting a sea invasion once without doing proper reconisance for what naval power the enemy had. A blunder further compiled by the fact that a trireme upgrades to a galeon which basicaly turns it from a warship to a weak transport.
I had a large fleet of triremes in stock, and I was soon able to I upgraded them all to galeons. Which all of a sudden made it possible for me to transport a rather large army, one big enough to try and invade a very pesky opponent that had been me bothering forever. I loaded up a ton of units, we're talking about lets see about 12 Galeons each 4 units so that's 48 units, mostly knights and pikemen, and about 8 crusaders with 8 trebuches.
Wanted a suprize attack that would cripple them, so I sent them out to three different points near their coast. Two places where they had Iron, and one was aiming for their largest coastal city. The majority of the galeons with the majority of the force was headed for the large city. I planted the ships just outside their zone of control so as not to cause trouble.
On my next turn I declared war, and moved my ships in towards their targets. The large fleet had to go into a bay of sorts so the stack didn't make it all the way where I wanted in the one turn so I could unload. I got greedy here actually, because I should have made for the the closest point. But I hadn't seen any naval power for a while so I took my chances. Bad mistake. On their turn a ton of frigates came out, and took out nearly half that stack of galeons. And other frigates came out and decimated my other stacks. Good thing at least those other I had emptied onto land during my turn.
So I lost a large chunk of my main force, and ton of ships which basicaly left the army that had landed safely stranded there on enemy territory. I continued for another round out of curiosity, landed my force that was left, lost almost all the remaining galeons on their next turn and there it was.

One big invasion foiled, and a large chunk of my offensive armies stuck on foreign soil, with no hope of bringing over the reinforcements waiting fast enough due to lack of ships now. Not that I'd try it anyways knowing I could loose them all in transit.
I didn't play much after, just went back and restarted from the last safe save.
