Ok, I have a theory - let's see if we can get some others to test it.
 
We almost never had THAT problem any more (used to occasionally) However, a similar issue we have, is that after entering a line - that is, the last Enter has been completed and normally any Desc2 lines and/or Kit Component Lines would be pulled in - we get the "Use Default Ending Lines?" dialog and then the SO is saved. When you go back in, only the first line of the item that happened with is there. It almost always happens after you've entered several lines (the earlier ones are still there and fine), but no pattern that we've noticed as to how many lines.
 
Our main order-entry person seems to think that it happens if you are going through the line too quickly. So I just did some testing.
 
The very first time I just pressed and held the Enter key after getting to Ship Qty, I got the error. Not only that, but I had a very very strange thing happen. I guess I held down the Enter key too long and some how got THREE SOs - the one I was entering, one for the same Customer with no lines, and another empty one, #0 with a date of 00/00/00.
 
These were all created BEFORE I got the Save sequences. I had stopped pressing Enter as soon as I saw the "Use default ending lines", and then Entered one at a time, and I had the Freight, and Print? dialogs, and then a "Save this SO even though there are no lines on it?" followed by "Default ending lines for SO #xxxxx", Freight and Print? for the other SO. Then another round for SO #0.  
 
Somehow the Customer selected for the first SO was still in a buffer or something, as that's who the two additional SOs are for - one with no line items and one with no number...
 
BTW, we don't usually just hold down the Enter key - I've done some further testing and I would say that if you allow less than 1/4 second or so between key strokes that's when the problem occurs (not the number 0 SO, but the saving and closing of the SO with only the first line of the last item saved).
 
So... I suspect all these problems have something to do with a buffer not being emptied fast enough. Anyways, give it a try and see what kind of results you get.

The whole architecture of SO-A has changed since the version you are on.  In your version, the header and line item entry are actually separate programs passing parameters back and forth and if the parameter gets dropped, it closes the program.  The current T7SOA is all one program so it isn't passing back and forth the same way.  If David is using T6SOA then it is still the 2 part program.

Quote from David Waldmann on 08/12/13 at 11:21:06:

 
 
*The problem I detailed with going too fast. We had the problem Val is talking about in a non-GUI version (I couldn't say at this point if it was BK or T6).