Thanks, all.

I tried to come up with some plot-lines for this, but they fell over at
the first hurdle.

A system *with* a gas giant but which is listed as not having one. Some
trader has found this out and is using the system to refuel and hence to
short-cut some trade route. (One would think that telescopes in close-by
systems would detect this, but you never know.)

Or, the opposite. A giant is listed but doesn't actually exist. Ships
Jumping in can only refuel at The Horrendously Expensive Fuel Monopoly
Inc (tm). (One might think that word would get out pretty quickly about
this.)

Jonathan