Currently I've found myself running two D&D campaigns in the same setting. My plan was to only run a single game, but I had so many requests I ended up splitting it into two, because of this I only had one actually campaign idea plotted out so I figured I'd run the same adventures side by side. I'm no fool and I knew eventually the games would diverge wide enough that eventually I would have to start making two different campaigns, but I greatly underestimated just how fast that would happen...
After only one session I have a single group up to 2nd level and getting ready to venture out into the world while my other group is now about a thousand gold in debt to the city and having to work jobs no one else wants to till the debt is paid. The funniest thing about this situation is it wasn't due to players trying to side step the plot or throw a wrench in things, but the rolls around a single trap. Both groups have characters able to find and disable traps so while I figured what is found and what becomes a threat might change that difference wouldn't result in this much of a swing.
I short version is while the first group fully explored a set of ruins and were able to level the other somehow took the only path (which includes randomly picking one of over a dozen paths and finding secret doors) that almost instantly took them to the last section of the area. This wasn't too bad as everything was set up to be changeling but not impossible. The problem came in the hallway after the big bad which was there for if the heroes wanted to explore deeper after their job was already done.
While the first group also went down these halls for them finding the traps within were a breeze. The second group wasn't as luck and despite everyone looking for them and the DCs not being extremely high no one could see the gauntlet of swinging blades coming their way. In a single roll two heroes were dead and a third lay dying. The fourth stepped up, helped save the dying hero and dragged the other two all the way to safety. Heroes were able to be resurrected, but even with all the coin they could find the best they could afford was a 10% down payment. And to top it all off even after coming back to life one now no longer has a leg.
The morel of this story? don't plot adventures further than maybe one ahead of where you are. Sure know what the plot is and if there's villains out there have their plans and what they might be doing on their own, but never underestimate the change a single dice roll can have on an entire campaign.
I'm about to be running a Savage Worlds campaign for some people soon that I have already run from start to finish with my regular group. I wonder how vastly different that will turn out.