The README says that the output file "will take the base directory from the first file that passes through it". In practice, the output file [takes its base directory](https://github.com/contra/gulp-concat/blob/c179a8c491047eb980459fea6d0e2fd65d6d0ad1/index.js#L85) from [the most recently modified file](https://github.com/contra/gulp-concat/blob/c179a8c491047eb980459fea6d0e2fd65d6d0ad1/index.js#L57-L60) that passes through it, not the first one.
The README says that the output file "will take the base directory from the first file that passes through it".
In practice, the output file takes its base directory from the most recently modified file that passes through it, not the first one.