In some formats, the file format itself is the only thing specified, and users are free to write their own software to read and write the data files. This allows customization and streamlining, but it also opens up the possibility that the user will get it wrong, that the files thus produced will advertise themselves as being in the format when in fact, they do not conform.
This problem can be substantially reduced, however, by the use of some sort of checkout utility to verify a newly created file.