cf-execd dies on Syntax errors were found in the main policy file

I upgraded all my CFE instances to 3.6.
I enforced the new protocol on all of them by adding protocol_version => "latest"; in promises.cf and update.cf

I have a simple cron that checks if cfengine is running and alerts me if it's not.

On 2 of them (we have hundreds), I got the alert, I sshed into them and I saw cf-execd not running:
<pre>
init(1)─┬─/usr/sbin/munin(24266)
├─atd(689)
├─cf-monitord(23101)
├─cf-serverd(23105)
├─dbus-daemon(599)
├─dhclient3(406)
[...]
</pre>
As you can see cf-monitord and cf-serverd are still running, but not cf-execd.

Just before sending the alert, in syslog, I found those lines:
<pre>
Jun 21 19:13:16 recommender cf3[9467]: Syntax errors were found in the main policy file
Jun 21 19:13:37 recommender cf3[9238]: Syntax errors were found in the main policy file
</pre>

Restarting CFEngine fixed it, but it's scary to see it crashing like this.
The weird thing is that I have hundreds of server applying the exact same policies and classes and it's only happening on 2.