While monitoring cron execution under one big site I've noticed that the execution was dying after some undetermined period (happening in different steps of the cron execution).

After some research, I think I've traced the cause of the problem. Explanation follows:

1) admin/cron.php sets time_limit to 0 (unlimited)
2) forum_cron is called and time_limit is set to 120
3) cron execution continues and, after 2 minutes, dies!

And this can be the cause of a lot of problems because some parts of the cron execution are never raised (clean-up tasks, backup...).

There are two possible solutions:

1) Forbid the use of "custom" time_limits within module/block... cron functions.
2) In the admin/cron-php set time_limit to 0 after each module/block... cron call.

Note that this is a problem since, at least, Moodle 1.7 ! And seems important!

Ciao