Expect the Unexpected

One of the first things you learn as a GM is that the players will absolutely wreck all of your plans at the first opportunity.  You can never make a plan that takes into account every possible action by your players.  This is part of the beauty of tabletop roleplaying games.  This freedom is simultaneously one of the best things about being a player and one of the most difficult things about being a GM.  As a GM, how are you supposed to deal with it?  How can you expect the unexpected?  Today, we're going to look at just that.

We'll start with an anecdote:

My sister recently prepped and ran the first session of an urban adventure for two players.  The party's mission was to recover a valuable statue from the house of a wealthy merchant on behalf of its rightful owner.  She made it a very open-ended adventure with lots of room for the party to make and enact their own plans.  She expected the party might attempt to pull of a stealthy night-time burglary, try to bribe one of the guards, or simply rough up and intimidate the merchant.  She mapped out the merchant's house and came up with a system for determining where the guard would be at any given time during his nightly rounds, and where the merchant would be in town or in his house at any time throughout the day.  She made a list of DCs for all the different ways they might sneak into the house.  While carrying the statue, the DCs are increased by 5, and failure by 10 or more means the statue gets dropped.  All in all, it was a pretty solid set-up for a heist.

The players, of course, had different plans.  In broad daylight, they bluffed their way into the house to discuss tapestries, then cast a spell that let them search a room without being inside it and immediately found the safe in which the statue was hidden.  They bluffed their way into the study and accidentally disabled the alarm.  The skald summoned a dire rat in the streets as a distraction while the inquisitor disguised himself with magic, and they walked right out of the house with the statue.

I'm pretty sure adventurers would rule the world if they didn't have that nasty habit of getting eaten by dragons.

What can we humble, god-like GMs do in the face of creative PCs and their outside-the-box plans?