Design patterns·The named ways it goes wrong
the old implementation is still there behind a flag nobody has turned on since the migration two years ago.
Lava flow
Also calledlava flow anti-pattern, fossilised code
Dead or obsolete code that has hardened into the codebase because nobody is confident enough to remove it. It grows out of unfinished migrations and abandoned experiments, and it costs on every read, every search and every dependency upgrade. The cure is deletion plus version control, and the reason it does not happen is that nobody is rewarded for it.