I didn't test this 100%, but maybe you can help me.

When I play online and I go to buy 2 items next to eachother (tower damage and health for example), I usually click them quickly one after the other.  I haven't payed much attention to how much gold i have but afterword I look at my gold and notice i have <20 left.  Now, dis this because I had exactly enough gold, or because I'm clicking it too fast and the server isn't updating my gold after the first purchase and givine me credit because I had enough before subtracting the first purchase.

If that made no sense to you.  Go buy 2 citadel upgrades, within quick succession of each other, with only enough money for one.  Do this online to factor in latency.  Then come back here and tell me what you find.