The A300-B4 when converted to cargo had a minimum weight that required some ballast or ballast fuel when empty. This we were told was the lowest weight that the aircraft was test flown at and therefore any lower weight was outside the certification envelope.
This limitation meant that when flying from Sharjah to Stansted empty, a fuel stop was required as around 4 tonnes of fuel was ballast. It took a little while to get ops to understand that if we could load some ballast we could do it non-stop.