I have a task scheduled every day at 3:00, on a Vista 64 machine. Most of the time it starts at 3:00:00, sometimes 2:59:59, which is fine.

But from time to time the task is launched at a completely arbitrary time, such as 12:50 or 18:30. Sometimes I see the task's window pop up in front of me while I work. It's very IO-intensive so there is a reason why I run it at night.

Here is an example from one month ago. It shows the task was launched at about 03:00:00 on 10/30, 11/01 and 11/02, but at 23:33:05 on 10/31. I am sorry the TaskScheduler screenshot is in French, but I have no way to change that.

Task Scheduler logs

The file c:\Windows\Tasks\SCHEDLGU.TXT indicates that Task Scheduler was up and running at that time (again, dates in French locale, sorry):

"Service du Planificateur de tâches"
    Quitté à 15/09/2011 11:58:50
"Service du Planificateur de tâches"
    Démarré à 15/09/2011 12:00:01
"Service du Planificateur de tâches"
6.0.6001.18000 (longhorn_rtm.080118-1840)
"Service du Planificateur de tâches"
    Quitté à 08/11/2011 15:10:19
"Service du Planificateur de tâches"
    Démarré à 08/11/2011 15:12:35

I see absolutely nothing relevant in the TaskScheduler logs that may indicate why it was launched such an odd time. The machine runs 24/7 and is never rebooted except when Windows Update asks for it. It runs an extremely limited set of third-party software, my work environment consists of little more than Visual Studio and MSYS. It also runs Linux in a virtual machine that performs regular tasks (this is not really relevant but it shows that the machine never goes to sleep either).

This person has exactly the same problem. The answer from Microsoft (destroying and recreating the task, or using another user account) wasn't very helpful to someone who would like to understand the problem.

How would I go about investigating what is happening?

  1. Are you sure that there is no other condition/trigger that might fire the scheduled task? For example there might be another user that manually triggers the task,if you allow so.
  2. Do you know the running time and if you have any conditions to run it ASAP
  3. You might also be subject to this bug which basically shows you correct repetition information but that's not the case

For further investigation you could get some sysinternals tools like Process Monitor and wait for the arbitrary triggering of the task to be logged. There's also a blog post in msdn that does some analysis on a task scheduler bug that stuxnet exploited.

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