The scrollbars can be set to a negative crew count. This can result in a negative, negative, score (but two minuses don't make a plus in this case).
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I noted that you can start a mission with a negative crew count. In addition, the ship's captain was present.
EDIT: I've identified that this is being caused by the moving UI (Ref: viewtopic.php?f=19&t=3924)
Still, locking down the variables to positive numbers only would be good practice here.
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