Cron validation prevents special strings such as @reboot
A recent change to the Cron package introduced field validation. Although the UI specifies time examples, some users may have entered special strings (e.g., @reboot) in the minute field, while leaving other time fields blank.
@reboot root sleep 300 && dosomething
The FreeBSD man page for Cron
@reboot Run once, at startup of cron.
@yearly Run once a year, "0 0 1 1 ".
@annually (same as @yearly)
@monthly Run once a month, "0 0 1 * *".
@weekly Run once a week, "0 0 * * 0".
@daily Run once a day, "0 0 * * *".
@midnight (same as @daily)
@hourly Run once an hour, "0 * * * *".
@every_minute Run once a minute, "/1 * * * *".
@every_second Run once a second.
@reboot appears to be the only non-time special string. The others could be written long hand.
Perhaps update the UI example, and allow one exception for @reboot?