Slitherine

In the Moscow 1943 battle a Sovjet anti-tank unit had to retreat to a frozen lake hex. In the enemy turn the unit did not move. Next turn the weather changed to clear and the lake was water again, but the anti-tank unit was still on the water! I would expect a same treatment like a plane with fuel shortage. Why isn't that so?

I've seen this before. It was very annoying when it was an enemy unit, but a relief when it was one of mine.
It seems that if you move onto ice and the ice then melts, you don't necessarily sink but maybe drift around on a decreasing ice floe until you reach the bank.

However, I'm fairly sure that I have seen units lost when on ice, so maybe it's something to do with movement type.

I can picture that. It was a (light) anti-tank unit and not a heavy tank. But it would be better if the game have clear rules about this situation.

Thawing takes a bit of time so it makes reasonable sense to allow units a chance of escape. If a unit is only allowed on a hex because it's frozen and wouldn't otherwise be allowed there (eg tank on a lake) it would make reasonable sense to treat it as a forced retreat - the unit would then be lost if there were no free hexes it could move into, but otherwise could escape.

But that's me thinking about it, and I don't know if that's what happens.

Well, this sort of thing happens because of the relatively simple movement system. The game only checks movement cost when a unit tries to enter a terrain hex. A unit can easily occupy a hex it cannot enter (some bonus units are even spawned in an inaccesible hex to make sure the player gets it).

So if a unit enters a hex legally, and then the terrain changes, the unit can remain in the hex without trouble. If it wants to move away, it simply pays the movement cost of the terrain it wants to move into. So if the unit becomes surrounded by hexes it cannot enter, it is effectively stuck in place.

Thanks ThvN.

My ice floe theory wasn't too far off!