Leaking or using to many GDI objects/handles. Those could cause a resource heap shortage. You might not be able to reproduce because your users might have other GDI resource heavy programs running or use Terminal Server in which case they have to share some of the heap with the other users. See System Error. Code: 8. Not enough storage is available to process this command
Here you can read about the Desktop Heap Monitor tool to diagnose desktop heap problems.