[Sharepoint 2010 - Error Fixing] The file has been modified by ... unknown

I got the following error message in visual studio during programing :

"The file has been modified by ... unknown"

It was because i called two times (by error) the item.update(); method.

This could happen as well when you code an event receiver (sometime happening when you do something on a list, web, site....) and you use item.update(), in order to fix that :

•  call and disable event firing before your code with : base.EventFiringEnabled = false;
• update your item by calling item.systemUpdate(false); 
• enable event firing with : base.EventFiringEnabled = true;