It was a scrap metal pricing and tracking system. It was written in BASIC and ran on an IBM 5110. It took me a week to write and test. I asked for $50 to do it, with $10 up front.
I got my $10, but I never got the rest despite my "customer" constantly praising my program and promising to send me a check for the balance.
That incident was the most valuable lesson I ever learned about writing code.
(BTW: $40 x 18% interest x 28 years = $4119 they now owe me!)