If you had to fold the $5 bill and force it into a slot, someone (usually drug addicts) can come by later and pick it out of the slot, using a variety of means. I live downtown and I have seen it done many times. Parking lots are aware of this and will give you credit if you report it to them.

BTW, if it's not city metered parking, there is'nt much they (Diamond, Key, U-park) can do to make you pay. Don't lose sleep, just give them a call if you are concerned about the ticket.
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Whoever undertakes to set himself up as a judge of
Truth and Knowledge is shipwrecked by the laughter
of the gods.

-- Albert Einstein