Software patent
A patent on an idea carried out by a program, which means you can infringe without ever having seen anyone else's code.
Copyright protects the text of the code; a patent protects the method itself. Independently arriving at the same algorithm is still infringement. In the US, the State Street ruling of 1998 opened the door to patenting more or less any business method in software form. The Supreme Court reined that in with Alice v. CLS Bank in 2014: an abstract idea does not become patentable by running on a computer.
Europe is formally stricter. The European Patent Convention excludes computer programs "as such", and the European Parliament voted down a software patent directive by a crushing margin in 2005. In practice, patents on "computer-implemented inventions" are granted anyway. The most notorious example is Unisys's LZW patent, which made GIF legally hazardous for a decade.