I worked on a project that depended on an open source but deprecated/unmaintained Linux kernel module that we used for customers running RHEL[1]. There were a number of serious bugs causing panics that we encountered, but only for certain customers with high VFS workloads. I spent days to a week+ on each one, reading kernel code, writing userland utilities to repro the problem, and finally committing fixes to the module. I was the only one on the team up to the task.

We couldn't tell the customers to upgrade, we couldn't write an alternative module in a reasonable timeframe, and they paid us a lot of money, so I did what I had to do.

I'm sure there are lots of other examples like this out there.

[1] Known for its use of ancient kernels with 10000 patches hand-picked by Red Hat. At least at the time (5-10 years ago).