When astronaut/cosmonaut mood is updated in start.cpp, the 'naut's mood is directly modified, with all modifiers being applied to their mood before it is eventually limited to the [0,100] range. If the mood variable wasn't an 8-bit value, this wouldn't be a problem - but mood is stored in an 8-bit variable. While 8 bits can safely hold the [0, 100] range of legal moods, it can't hold the pre-limited temporary values.

Consequently, a happy astronaut going on a successful mission can find their mood value wrapping over into negative numbers, leading to two entire flight crews resigning from sudden bitterness during their moment of triumph. Sigh.