One such screw up happened today. A PC wanted to created a herd of restive horses using Phantom Steed, exhausting all his reagents in the process. He then wanted to make the horses charge a barn where a hag was hiding in. According to the rules, that was not possible. But I screwed up, so it was. 

What resulted was an extremely fun encounter where the PC drove the herd into the barn at a gallop, crashing through the flimsy building, bringing down the roof and the barn on top of the hag. The hag had to then extricate herself from the rubble, and then discover herself surrounded by hostiles all around her. 

The PC then charged her atop a Phantom Steed, did a backflip from the saddle and twisting in mid air, did a powerful lash that pushed and prone the hag (a fighter's Bull Charge). I demanded an AP for that, but for that cinematic and heroic scene, I did not cry foul. 

In fact, I realized, I encourage it.