It is hard to imagine such a game. Here is an example from wikipedia:
Suppose you are playing a game of chess against an opponent who will be paid some substantial amount of money if a particular event happens (an arrangement of pieces, for instance), but you do not know what the event is. In this case you have perfect information, since you know what each move of the opponent is. However, since you do not know the payoff function of the other player (which will affect its behavior even if it does not alter your own victory conditions), it is a game of incomplete information.
Do such game situations exist in real life?
What kind of equilibria can we observe?
Is there a dominant strategy that could be adopted in such a game? Is there a way to maximize your gain even though you don't know all the details?
Still in the chess example, you might start forming some beliefs after a few moves, if you notice for instance that the opponent is always sacrificing his pawns for nothing, maybe in his desired position he needs to have a certain amount of pawns... Or if given the opportunity to exchange queens and in a classical game of chess it would have been advantageous, but he doesn't do it, assuming he takes rational decisions, perhaps it is absolutely necessary for him to keep the queen in order to win with the altered rules.
You might assign certain probabilities to the different possible goals of your opponent and update them with every move he makes.