Something like a remnant of a system that originally had a purpose, but has since lost its purpose, to the point of having a negative impact on the current system.
Think of something like an outdated technology, that currently has a range of issues. In it's prime, it was a good choice, but now it is causing more issues than it solves.
Real world examples:
'The system contains x technology, likely a [insert word here] from when the system was first designed.'
'The human body still has a [insert word here] tailbone.' (nowadays causing more problems than it has benefits)