Ask HN: What critical legacy system at work is everyone afraid to touch?

I recently inherited an old phone and call centre system at work It runs on Elastix/CentOS, a custom windows app, MySQL, GSM gateways with analogue devices and static ips, with almost no documentation at all. The strange thing is, it works really well. The problem is that nobody fully understands every dependency anymore and changing the wrong thing could affect a large part of the company so for now am documenting everything, improving the backups virtualising it and isolating it before attempting replacment. Has anyone else inherited a critical system that everyone was afraid to touch?