A major river case to think about

[Reconquista]

I would say that the rules cover this and the Panzer could n enter the hex:

5.3 Friendly and Enemy Units in the Same HexA unit may not enter a hex occupied by one or more enemy combat units unless it is conducting an Assault, 

Since neither side can cross the river no assault can occur and thus the Panzer cannot enter.

Another nonsensical PG rule..

Checkout the disorganized Infantry Attacks 2nd edition rules for MANY more examples