What Kinds of Things Would Cause Units To Ignore Orders?

There is something weird still in my map where sometimes units wont obey commands until they have completed an existing order.

And example of this is: I may right-click on a unit to have a selected unit walk over to that clicked unit and if i tell it to stop after to cancel that previous order or any kind of order at all... it wont obey. The icon doesn't even flash like it's registering as a button push. and that unit will regain control of the command card/or hotkeys for those commands the moment he arrives at his destination to complete the order.
Or if he is told to attack something, he will attack it until it dies, never allowing me to stop him. I guess he's determined. heh.

what would cause such a thing?

Could having too many units on screen cause this? the game doesn't feel like it's lagging but there are quite a few units visable on map compared to most maps. as well as doodads.

that's the only thing i can think of.

it is a random occurance. I could play for a while then and not see anything unusual most of the time.

Are you holding down shift rather mistakenly or knowingly?

It can be caused by bad use of morphing spells (Metamorphosis, avatar, chemical rage)

If you are using one of them, make the morphing type not "uninterruptable"