ferry companies do this
a year or so ago I was going across the channel and when booking ferry tickets there were all kinds of specials etc and it was often cheaper to buy two returns and throw half of each return away. Don't ask why.
But on the websites was a big warning saying if half a return was thrown away thus turning the ticket into effectively a single then the ferry company reserved the right to charge your credit the extra the single would have cost you! And looking on the web the ferry companies regularly did this to people's cards, it wasn't an empty threat.
G