Some Obscure Monopoly® Rules Explained

Jail and doubles

If a player goes to jail his turn immediately ends even if doubles are rolled. You do not roll again under any circumstances.
 
If a player gets out of jail because doubles are rolled he does not go again. In a sense the "power" of the doubles is used up in saving you the $50 fine. The exception to this is when a player pays his $50 upon going to jail and then rolls doubles on his way out on his next turn. This is because as soon as you pay your fine you are out of jail and "just visiting" so the doubles are not necessary to get you out.
 

Income Tax

If a player passes GO and lands on income tax in a single turn his $200 GO money is included in his total worth.
 

Building Hotels

A player does not need to physically have four houses on each property in a colour group before he can buy hotels. However, there must exist enough houses in the bank in order for that person to do so. If there are not enough houses, the hotels cannot be bought.
 

Utilities

You do not role again when you land on a utility to determine the fine. The fine is base on "amount shown on dice".

Quitting

If a player wishes to leave the game all assets are turned into the bank. The bank then auctions off any properties. Under no circumstances can a quitting player give over their assets to another player. Note, there is nothing to stop any player from selling their properties and any "Get Out of Jail Free" card to another player at any price (say $1) and then quitting. Remember, though that no player can give cash or buildings to another player, these go to the bank.
 

Mergers

Mergers are not allowed in the official rules yet there are players that will always insist upon them. If two players wish to begin playing together then have one quit as above and remove that player's token. Players cannot simply combine their assets.
 

Immunity

Under no circumstances can a player offer immunity against rents to another player.
 

Watching your Properties

It is your responsibility to watch if someone lands on your property. If someone lands on your property you have until the end of the next player's turn to notice it. This player's turn ends when the player after him throws the dice.
 
Example: June lands on Park Place, which is owned by Bill. June plays dumb (which is what she shoud do) and passes the dice to Mary, who roles completes her turn and passes the dice to Paul. Before Paul roles Bill notices June on Park Place and announces such. Does June have to pay up? Yes. Bill has to the end of the next player's turn to notice June. If Paul had rolled the dice, though, Mary's turn would have ended and Bill would have been out of luck.
 
The rule works this way to prevent players from just quickly passing the dice on while the property owner sneezes.

This page is based on this page:

http://www.geocities.com/Area51/Corridor/7111/extend.htm

I've added some commentary of my own, and done some editing.

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