My worst purchases is when I buy some plugin that I already have but forgot to check my inventory.
I seem to do that at least once a year.
The (tell myself) upside is that it was an irresistible item at the time, twice.
I'm going to commit new purchases to a $50/month limit (unused rollover allowed).
That of course does not include upgrades. They will be limited to $150/month with no unused rollover.