Quick way to lock up your shell

[Bart Schaefer <schaefer@xxxxxxxxxxxxxxxx>]

schaefer<501> FOO==(cat)

The shell is now hung forever waiting for a "cat" that will never exit,
because cat's standard input is neither closed nor connected to anything
(expect to the parent zsh, which isn't writing to it).

This happens at least as far back as 4.2.0, so it's not a new problem.

Similar things can happen with <<(cat) except in that case the shell is
not locked up, it has a background job of whose status it loses track.
I haven't figured out exactly how to reproduce that one, but once it
gets that way even killing the job externally will leave zsh believing
that the job is still active (so e.g. "wait" hangs until interrupted).