at people who complain that the glitch isn't working, it's not meant to increase your money. the money goes all down into the negatives, it just doesn't show the minus sign. what the glitch does is increase the firing speed of the tacks by a ridiculous amount (it works on some other upgrades too). on a related note, it doesn't seem to be a glitch really, rather an intended way of cheating. note how the firing speed indication says 'very fast' and then 'hypersonic' after upgrading a few times.
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