When you use setValue, it will trigger an onValueChanged event on the control and an onUpdate event on the databasenode. If either of those script calls ends up calling another setValue call on the same control/node before control is returned to the client, then the setValue script call is aborted and the warning message generated.
Usually, this is indicative of a circular update. (i.e. setValue -> onUpdate -> setValue -> onUpdate -> ...)
Cheers,
JPG