So, I had something similar happen with a cup of tea ruining a watercolor piece. The stains dried into these interesting patterns that I ended up scanning and using as backgrounds. Sometimes the best tools come from happy accidents.
Have you ever read about wabi-sabi, that Japanese idea where flaws are part of the beauty? I saw a piece on it and it stuck with me. It's not about ruining your work, but learning from when things don't go as planned. I agree planning gets solid results, but sometimes a mistake pushes you in a cooler direction you wouldn't have tried. It's like the accident forces a creativity you can't schedule. How do you decide when to stick to the plan or roll with a mistake?