Install Base Transaction error - The Inventory Item (......) doesn't exist in the Inventory Organiza

We have suddenly got hundreds of these failures since 7 May 2012 in Rel 11.5.10 environment.

It relates to items that have been set up for a long time, they have been assigned to all the Organizations on these error messages.  The item numbers haven't been changed in any way since last year

All the items involved are serialised, but not all serialised items seem to be failing (only 24 separate item numbers and we ship far more than 24 serialied item number)  But all 24 are from same Product Line

The messages relate to both RMA and Shipments.

The items are all serialised and if I search for the serial number in Inventory it says it is not there.  In IB it says it is internal in location awaiting shipping (99.SHIP).  All logical inview of the error message.