The hairline shows how long I've been in IT
My co-worker Shaun Cassells made an important discovery when we began patching with SMS 2003. When you advertise the patch update packages, you must always set up a recurring execution. The logic of the patchinstall process requires this if users are permitted to postpone the updates. See KB842717. If you have some update packages that allow postponement and others that don't, you're best off being consistent and using recurring advertisements for all of them. That greatly reduces the chance of creating this error. If you have DSUW create the advertisements it will always create a recurring schedule.
If you have an update that you really only want to run once, such as for pilot testers, just select a monthly recurrence schedule and delete the advertisement before the month is up.
In general, it doesn't hurt to have updates run on a recurring schedule. The update will rerun the scanner, and then only apply any updates included in the package that the scanner found were applicable. Once a set of updates are applied, rerunning that package will do nothing unless changes to the computer made one of them applicable again.
Scanners should also have a recurring schedule that assures they will always be run before the update package. Although the update package repeats some scanning functions, that does not replace running the full regular scanner. The regular scanner detects updates that are applicable or installed. The scan function within the update verifies if that information is still accurate, but won't perform the initial detection.