Software doesn’t exist in a vacuum — the environment and its inputs and outputs change over time. So it’s likely to break at some point and will require maintenance.
I’ve got a now-ancient static webgallery generator that I’ve tweaked and used for more than a decade. I enhanced it so that it creates animated gif thumbnails for movie files using a combination of transcode and mencoder. Recently, I added an MP4 movie file along with my photos. The web gallery generator chugged away longer than usual, and I gave it no notice — until it filled up my hard drive.
It wasn’t expecting to encounter an additional movie file format, and when it did, it went with the default of using ImageMagick to generate the thumbnail, instead of using my alternate solution for movie files — and ImageMagick filled up my hard drive with a giant temporary file.
So I edited the Perl-based web gallery program and added .mp4 and .m4v files to the list of special cases to be handled separately. It will work until the next time another new movie file format is encountered, and then I’ll need to maintain it again.
Nearly all software is that way — it must be maintained, or else it rots.