Comment 2 for bug 107209

This may be a little more informative, if baffling (again, also posted to zsh-workers thread):

$ bash -c 'ulimit -t 0; ulimit -Ht; while :; do :; done'
0
Killed
$ bash -c 'ulimit -t 0; ulimit -Ht; (while :; do :; done)'
0
<loops forever>
$ bash -c 'ulimit -St 0; while :; do :; done'
CPU time limit exceeded (core dumped)
$ bash -c 'ulimit -St 0; (while :; do :; done)'
<loops forever>

We can see that for both hard and soft limits, the child is not stopped.
(Note that bash's default is to set the hard limit, unlike zsh which
sets soft. POSIX gives very little specification to ulimit whatsoever). But:

$ zsh -c 'ulimit -Ht 0; ulimit -Ht; while :; do :; done'
0
<loops>
$ zsh -c 'ulimit -Ht 0; (ulimit -Ht; while :; do :; done)'
0
<loops>
$ zsh -c 'ulimit -St 0; ulimit -St; while :; do :; done'
0
CPU time limit exceeded (core dumped)
$ zsh -c 'ulimit -St 0; (ulimit -St; while :; do :; done)'
0
CPU time limit exceeded (core dumped)

Note that zsh's behavior seems to depend more on whether it's hard or
soft, rather than whether it's the child or the original.

I can't make heads nor tails of it, but /something/'s definitely screwy.