I tend to do MANY blunders with oversea transportation. Not the biggest, just very common. One that comes to mind is when I'm attacking in another continent and need reinforcements urgently, but I've been preparing for it and have a bunch of transports on the closest city, carefuly placed so that they can cross to the other continent in exactly one turn. All are fresh and ready, but there's a stray transport built in another city, who just arrived and has only 1 move point left. N.P, I load reinforcements on the fresh trans, and workers on the stray one, as they aren't as urgent. Then wake up everyone and send all transports and escorts at once, just to see that they were solidary with the strayed and only moved one tile
And this. I'm bringing a transport to be loaded with marines and assault a close island. Then I think if I leave it just outside my city where marines are waiting and jump them on it, I'll save one move point. So I do, and then send it to attack. Good! That tile I saved alowed me to arrive right next to the city to be attacked... just to see that by jumping on it the marines spent all their move points and now they need to wait the next turn.
Oh I loved this! On my last game I had just discovered steam power and my advisor tells me I should build the iron works. Great! But the city has mostly sea tiles, as the adjacent city is using all the land tiles. N.P, I switch tiles, send the workers on the adjacent city to work the sea and make these work the land. Now I have many shields, build the iron works and everyone is happy... except me, when I remember the adjacent city had built the military academy. Let's see how long it takes to build an army out of fishes