Yes, mixing in the tank will cause that. I was stranded and out of gas once. Lucky me, I was right next to an auto parts store and a gas station. I could get everything I needed, but I forgot to turn the gas valve off before I put the oil in the tank and some of it ran down into my filter. I took the filter off and dumped it back into the tank and added a few drops more just to be safe. The bike ran fine and I got home.