Guido Ubaldus' proof of the existence of God (idea)
Actually, I think it proves that his math is all jacked up because Ubaldus is treating an infinite series like a finite one. Line 3 is the illegal operation. It is patently false when dealing with ellipses. The dots matter if you are going to perform operations for which the last number becomes important. Once he grouped the numbers parenthetically in line 3, the statement is only true if the series ends with a -1, so he uses a classic bit of polemic by changing definitions and counting on you to miss the substitution. Math proofs work for math and often predict things in the real world, but this is only useful to the extent that the conclusions can be tested.