I have not come across this before and just wondered if anyone had any
suggestions as to the cause.

I have a parent form, a subform and a subsubform for Invoice Number (VBA
Generated)
Linked to referenceNumber on the first Subform. which in turn is linked to
the subsubform by an autonumber. The order of the autonumber doesn't matter,
the value id insignificant. However, the data base I wrote worked very well
for 2 years but fell over today.

The autonumber value was up to 2500 ish and is a long int, today I received
a frantic call referencing an error, duplicate primary key failuer. It took
me a couple of minutes to work out that the autonumber had been somehow reset
at 40. Obviously record 41 existed so it gave the dup error.

I have over come this issue very easily but I wondered if you Guys had come
across this issue?

Kindest Regards

Mike B