Rule mistakes
This is a work in progress. It's going to get quite long. It's quite a common event the next morning for an email to go round the group saying "By the way, we got something wrong last night...".

Now this one isn't our fault. We were taught the game wrongly by the person who demonstrated it for us in Essen. The way we were taught was that player one plays all of actions that he wants to take. Then player two does all of his. And so on once round the table. The basic strategy became to hoard all of your cards until it was your turn to go last so that nobody could respond to you.

Lost Cities
How can you possibly play this game wrong??? We did, a few dozen times. We thought that you add 20 for an eight card trip before multiplying for handshake cards. The strategy became go for eight card trips at all costs, even if you score negatively on four other trips. We couldn't believe Knizia had made the scoring so one dimensionsl. Until we played a computer version and noted it had the scoring "wrong".

Mystery Rummy - Jack the Ripper
We deliberatly play this one wrong, as we can't be bothered with the fiddly voting rule. But we also got a rule wrong. We played that all gavel cards get placed in front of you. So once both Comissioner Resigns cards had been played, there's no way they were coming out again.

Princes of Florence
We actually got this right the first time we played it, and very badly wrong the next. In fact we got it so badly wrong that we had to abort. We got the 2 for 1 Prestige Points to cash multiplier the wrong way round. We were flying round the board and watching the cash pile up in front of us....

Puerto Rico
We got a few rules wrong here. We cheated the start player of his indigo. We forced anyone shipping to ship the commodity he had the most of first. We counted empty circles on plantations when refilling the colonist ship. The game didn't half end quickly.

San Juan
Two errors here. Two that we know about, anyway. We started by giving each person an Indigo Plant in the hand, not on the table. This made absolutely sure than nothing was produced in the first turn. And we completely missed the fact that discards are face down, despite it being emphasised in the rules. Update - found a third error! We didn't bother reading the two player rules (heck, we're gamers, we'll figure it out - how hard can it be?). So we didn't notice that the governor takes two picks each round. It did seem a little odd that we alternated two turns each, now you come to mention it...

Street Soccer
We allowed movement points to be split between several players. Although this led to a cagey came, it encouraged a very defensive style and hence no goals.