No, this hasn't happened in one my games yet. I just thought this would be an interesting thought puzzle.
There is a major river running across the battle field. The blue side, Poles in this example, are on the north bank.
A tank, German PzIIIb in this case, moves onto the south bank.
What is the situation now? Clearly they are not in contact, so it doesn't seem to be an assault hex. Can they just fire at each other across the river? Could the Poles take opfire as the Germans are moving in the hex? (They are on the other side.)
I think they should be treated as in adjacent hexes, until someone tries to cross the river.