If you sell a commercial compiler or assembler, and some company starts selling a product produced with your compiler/assembler, and you know you never sold the

If you sell a commercial compiler or assembler, and some company starts selling a product produced with your compiler/assembler, and you know you never sold them a license, you've got yourself someone to sue. For a less successful/commercial product, it could also just be for the joy of finding it used in the wild. Similar fingerprinting techniques were also used by assembly coders as proof of copyright. When reverse-engineering to a spec, it's not unusual for routines, even long routines, as well as lookup tables, etc. to end up being coded the same, or nearly so, as the original software. This isn't necessarily infringing of copyright -- for example there are only a few sensible ways to check if a character is uppercase in ASCII. It can be hard to tell reverse-engineered and re-implemented code, from disassembled and reshuffled code (which is copyright infringement). If the suspect routine incorporates something like an idiosyncratic sequence of NOPs that serve no particular purpose, coincidence is a lot harder to argue.