When using the g_COINS variable to change the value of the player's Squares, subtracting it below zero doesn't stop the value from decreasing further, it goes beyond that; you'll have less than no Squares (but at least the game doesn't read it as "positive").
This may occasionally break things if exploited unsparingly, but as long as there's always a way to get more Squares, it shouldn't be too much of an issue. It still doesn't make sense to be able to have negative Squares, though.