I found a bug in the scripting, and managed to isolate it to a easily-reproduced gamestate.

With a little bit of effort, I can confirm that the bug is somewhere in the Market Board's function "findcardstoreturn()".

To reproduce:
- load up High Frontier 4 All Official.
- hover over a deck, and draw the correct number of cards so that one card remains on the market board. (for example, pick up 6 gigawatt thrusters, by pressing the '6' key)
- drag one of the cards onto one of the market board's return spaces.
- click the 'Return Cards' button.

You will see the card get placed underneath the remaining gigawatt thruster, and then there will be an error. The Return Cards button will no longer work.

A simple workaround is to, when you are returning a card, check if the market only has one of that type of card. If it does, simply return the card manually. If there are no cards remaining, or more than one, the button's scripting will not crash.