There are two items in my Zabbix installation that are checked once a week: item A on Monday at 3:45 (achievable by setting the main delay to 0 and flexible interval to 60 seconds at 1,03:45-03:46) and item B on Monday at 12:45 (1,12:45-12:46). Initially, I have misconfigured them: item A was checked at 12:45, item B at 3:45. Last week I corrected their flexible intervals, but item B was still checked at 3:45 today. The reason this happened is because Zabbix server does not immediately update item's nextcheck (and item's position in the queue) if flexible intervals change, like it does with regular delay.