Some crop circles have been discovered to be hoaxes, but the people researching this stuff claim some others do not appear to be. Frankly, I have no idea, but it would not surprise me if some crop circles appeared that were not hoaxes, and then people got into the act and started using the phenomena to create hoaxes.
That's happened with UFOs. There are extremely credible UFO reports such as the 1947 appearance of UFOs visually and on radar over DC which resulted in multiple scrambling of jets. But there have been hoaxes too, probably once the idea got out and some pranksters wanted to have a little fun.
Of course, the thousands of people that witnesses the UFOs over Washington and the military scrambling jets to intercept, the pilots first-hand reports, and the radar signals were not hoaxes.