Most likely nothing bad will happen. Although if you left it in the rain, and the live end happens to be in a puddle....well, if anything did happen you'll probably just trip the circuit breaker that the cord is plugged into. But if someone happened to be walking into the puddle...while barefoot...and decides to pick it up...

So while the probability is that nothing bad will happen, it's almost a complete certainty that nothing good could possibly result from it either. I have left many regular extension cords out in the rain with no accidents. I try and be careful, but sometimes an unexpected shower happens and I'm not at home to put everything away. So while I don't recommend it, it's not something i would worry a whole lot about.