To expand on the above a bit...
There are things that are scheduled and are getting run from time to time that aren't specifically set to run on a given time. Example: In the "\Microsoft\Windows\Diagnosis" section, there's a task called "Scheduled" that really isn't (scheduled). It ran for me on the 5th successfully at 11:41:31 pm, but isn't set to run again on a time schedule.
And there's a "Data Integrity Scan" in the "\Microsoft\Windows\Data Integrity Scan" section that's scheduled to run on 6/17/2014 at 10:32:58 AM and it's Disabled. I didn't disable it, and somehow I expect it will actually run.
There's altogether too much magic and hidden crap shipped in these systems any more.
Edited by NoelC, 10 June 2014 - 01:27 PM.