One of the most frequently asked questions by new designers is "how do I protect my ideas from being stolen?" It can come as a surprise to realize that it's not that uncommon for different designers to come up with very similar mechanics.
To prove this, I thought it might be fun to start a thread in which we could post anecdotes about coming up with a mechanic and then subsequently finding a very similar mechanic in a published game.
I have two examples that I can think of immediately, but I'm pretty sure there have been others.
One of my earliest designs was a game in which players were officials in a government. Each turn a set of "problems" would confront the nation, and players would play character cards who could solve the problem. For example, if the problem is "war", the "general" card would be the strongest solution. But each problem also had two characters who could "support" the solution. Shortly after coming up with this idea, I learned about the game "Krieg und Frieden", which has a virtually identical system. There are four different problems, and four different resources, and for each problem, each resource has a rating between 1-4. So for "War", "Knights" are very useful, but they're less useful if the problem is "taxes", for example.
Another example is my GDW archaeology game, "Profit and Provenance". In that game, I was exploring a concept of "semi-blind bidding", since blind bidding games like Aladin's Dragons were popular at the time. I came up with a system whereby the backprinting on each card partially telegraphs its value -- the cards were valued between 0-8, and each card had between 1 and 3 coin symbols on the back, with the number of coins corresponding to the possible value of the card. The game "Maya" came out a few months later, and it had almost exactly the same bidding system, with exactly the same denominations and nomenclature! It was really an amazing coincidence.
Some more general examples include "Shadows over Camelot", which, like my GDW game "Disciples" has a traitor mechanic, and "Karibik", which, like my pirate themed game, gives all players access to all pirate ship (instead of each player having his own). But these similarities are much more general; the ones above were much more uncanny.
Please share if you have similar examples!
-Jeff