Any number of unwanted or troublesome behaviours may ensue.

mark_j said:
… possibly is a bug. Even if the file system is junk, the driver should time out and allow the process to be killed. …

Depending on the context of an error, it's not unusual for an operating system halt to fail in response to shutdown -p now.

shutdown(8)

Ideally: things should be more graceful.

Realistically: it's sometimes necessary to force off the power.