Unknown error (security validation) when modifying user permission on a master page?

We have a inner master page wth custom css that our departments are "supposed" to be able to edit however it is not working that way.
I am the farm administrator and site collection admin.  I went to remove a user from the master page and received an error.  Can you tell me what might be causing this and how I can fix this...
The only workaround I have right now is to turn off security validation in Central Admin but I can't do this each time a user needs to add a user or add an object to a library...

System.Runtime.InteropServices.COMException: The security validation for this page is invalid. Click Back in your Web browser, refresh the page, and try your operation again.  
 at Microsoft.SharePoint.Library.SPRequestInternalClass.UpdateMembers(String bstrUrl, UInt32 dwObjectType, String bstrObjId, Guid& pguidScopeId, Int32 lGroupID, Int32 lGroupOwnerId, Object& pvarArrayAdd, Object& pvarArrayAddIds, Object& pvarArrayLoginsRemove, Object& pvarArrayIdsRemove, Boolean bRemoveFromCurrentScopeOnly, Boolean bSendEmail)    
 at Microsoft.SharePoint.Library.SPRequest.UpdateMembers(String bstrUrl, UInt32 dwObjectType, String bstrObjId, Guid& pguidScopeId, Int32 lGroupID, Int32 lGroupOwnerId, Object& pvarArrayAdd, Object& pvarArrayAddIds, Object& pvarArrayLoginsRemove, Object& pvarArrayIdsRemove, Boolean bRemoveFromCurrentScopeOnly, Boolean bSendEmail)