It is always fun when players go off-script. Sure, it means all your carefully prepared plans might go out of the window, and you'll have to do a bunch of fast-thinking improvisation. But it can also be a huge opportunity for the players to make a difference in their world.
Let me tell you of one such case, where a simple act of Chaotic Goodness led to ramifications the players could never have dreamed of.
The Adventure
The party were on their way to the frozen North, to retrieve the magical banner of a lost regiment (yes, it's the image at the top... I really did make that as a full size prop...). On the way, they passed through the Isvar Mountains and arrived at the grubby trading post of Valtraksalt, an outpost of the mercantile city of Magoran.
(Map created in Wonderdraft)
In Valtrasksalt, two of the party, Tostig the (chaotic good) battle priest of Thor and Celeste the bard (possibly the most utterly chaotic character I've ever had the pleasure of DM'ing) discovered a batch of slaves being loaded on a barge to be sent downriver. It was clear the local slaver, Mirin Gabadashi, had the militia in his pocket and more guards than the party could handle, so Celeste persuaded him to allow Tostig to pray and preach to the slaves before they departed. She took the opportunity to slip the least fearful-looking slave a knife while the guards were distracted.
The party then carried on with their adventure, arriving in the city of Magoran, where what I had expected to be a 10-minute resupply visit turned into 3 months of gaming as the party went totally and enjoyably off-mission following an assortment of red herrings. While they were there, Gabadashi sent them an invoice which started "Thank you for your indirect purchase of 50 mine slaves", which told them their ploy had been successful and the slaves had escaped (they never did pay him....)
The Aftermath
After many adventures in the frozen North, the party returned home, having gained several levels and quite a reputation.
Shortly after, they received a letter....
That was the hook for a whole new adventure, in a new nation which would never have happened if they hadn't passed a slave a knife. The players still proudly say that their actions changed the world and added a whole new country. Actions have consequences !