As described in the Nilsson's Aerial Ballet article at NilssonSchmilsson.com, Carl Emanuel Nilsson, Harry's great-grandfather, lost a fight to protect the patent for his "Aerial Ballet" device. The device allowed stage performers to appear to fly and "dance" in the air (think of Mary Martin as Peter Pan).

As you can read in the article, the defendants claimed that similar rigs had been used before in stage productions and they compared Nilsson's device to a lift used on farms for moving heavy objects. By demonstrating "prior art," the defendants were able to convince the court that NIlsson's patent is not valid.

I'm not an expert on such devices and I am also not a lawyer, but it looks to me that Nilsson made a mistake by making the patent too broad. I suspect he did create refinements that made his apparatus work better than existing ones. The rave reviews and popularity of his Aerial Ballet show is good evidence of that. These improvements should have been patentable on their own and a patent on them may have helped protect Nilsson from the copy-cat productions.