Error: Failed to update permissions during Impersonation Step in Workflow

Hi,

We are having the following problem when a workflow created with Sharepoint Designer 2010 which is configured to run when a list item is created or modified. The problem seems to occur randomly, out of 1900 items in the list (created/update numerous times) it has occurred 5 times and at different times for different users.

The workflow contains an Impersonation Step which only contains actions that set permissions on the item. 

This is production system and this workflow error is breaking the business process (expense claim), hence impacting us severely.

Date Occurred Event Type User ID Description Outcome 11/14/2012 8:01 AM Error System Account Failed to update permissions. Unknown error 11/14/2012 8:01 AM Error System Account An error has occurred in Expense Field Updates.

Can you support?