Freeciv

Many of our tilesets define sprites for extras, but don't mention them in extrastyles. base.output, base.airstrip, and base.buoy are common cases.

It looks like there is independent fallback for alternate extrastyles and the actual extra graphics. So rulesets like civ2civ3 have been getting away by falling back to from "base.outpost" to "base.fortress" for extrastyle, while continuing to use "base.outpost" actual graphics. Fortunately, the extrastyle has ended up matching the graphics.

However, a ruleset like 'alien' has "base.force_fortress" falling back to "base.outpost". If an attempt is made to use a standard tileset with 'alien', it bombs with

0: No extrastyle for "base.force_fortress" or "base.outpost".

This can be provoked by doing

freeciv-gtk3 -t isophex -r alien

and starting a game, and declining any prompt to change to 'alio'.

(It doesn't make much sense to allow differential fallback for extrastyle and the actual extra graphics, since the two are coupled. Probably we should somehow enforce that the matching extra style and extra graphics are used. Don't know how practical that is, particularly in the face of tilesets pulling in bits of other tilesets that may define sprites that the first tileset is not interested in.)