I owned Whitechapel and Old Kent Road, both with hotels. My friend owned Mayfair and Park Lane with no developments.
We traded and I gave a further two million dollars to him. Nothing was said about the fact this was an illegal trade because we didn't know there was a rule that you can't trade properties with houses on them.
Later in the game, another player asked if you could sell properties back to the bank. It was only during the search to that question we discovered that the above was an illegal trade which has affected everyone.
If the rule was unknown until later in the game, do you play on as no one had brought it up when it occured? Does the whole game become invalid? Do you play on or quit?
Is there an official rule about not being able to argue a rule once a turn has passed?