In game design, one of the holy grails is something called "emergent gameplay." Emergent gameplay is gameplay that wasn't designed, but instead emerged from the interaction of the various rules.
The "Grand Theft Auto" series is being held responsible for what can only be called a complete and utter excess of emergent gameplay.
"GTA" is back in the news after the Sex-Workers Outreach Project called on consumers to boycott the game because players get points for killing hookers. Now, I'm not going to address the fallacy that SWOP is operating under, but I am going to show how a few simple rules operating together can generate multiple and sometimes unexpected results. We'll start with the sandbox nature of the "GTA" games, and look at some rules that Rockstar could have integrated into their game.
Rule 1: NPC's have professions.
Rule 2: An NPC will have in their possession items related to that profession. (For example, a police officer will have a gun, a shopkeeper will have money, etc.)
Rule 3: Players can purchase items and services from NPC's. Doing so transfers funds from the player to the NPC in question.
Rule 4: NPC's can also purchase items and services from other NPC's. Doing so transfers funds from one NPC to another NPC.
Rule 5: The service that a player can purchase from a prostitute is a health refill.
Rule 6: Players can kill any NPC.
Rule 7: Killing NPC's raises a player's Wanted level.
Rule 8: NPC's with weapons will use them against a player that they see kill an NPC.
Rule 9: Killing an NPC causes that NPC to drop any items that they have on them.
So, let's see how what SWOP is complaining about came about. Prostitution is an NPC profession. (Rule #1) Prostitutes offer a service, so the only items that will be on them are funds. (Rule #2) Players will, through the course of the game, run low on health, most likely because of (Rule #8). If a prostitute is handy, a player can hire their services for a fill-up. (Rule #5) This transfers some funds from the player to the prostitute. (Rule #3) Finally, some players found that after using the services of the prostitute, you could kill the prostitute and get your money back. (Rule #6)
While the end result is not desirable by any stretch of the imagination, it's the end result of that chain of rules.
Likewise, if NPC's are constantly going into a restaurant and purchasing food from a clerk (Rule #4), that NPC would have more money at the end of the day than they would at the beginning of the day should (Rule #6) be invoked.
Of course, "GTA" isn't the only game to have emergent gameplay attached to it. "Quake" featured "rocket jumping," which was the result of two rules interacting: rockets created blast forces, and players were affected by those blast forces. Playing a game of "Monopoly" will often result in some heated sidebar discussions trying to swap properties for properties, etc. "Chess" has literally millions of potential game combinations, even though it only has a small set of pieces and potential movements for those pieces. Hell, even "Tic-Tac-Toe" has behaviors that emerge as a result of that most simple of rulesets.
We can never completely forecast what players are going to do with the rulesets we create. All we can do is make the rules. The player is the one ultimately responsible for deciding how he wants to play by those rules.