One such example could be the Microsoft Points - perfectly falls under your description. I can never remember the exchange rates, and they deliberately offset the prices so you always have residual points. For example, the minimum points you can buy are 400 ($5) but the cheapest item is 80, then 240, then 360... so you will never be able to spend ALL of them entirely (barring many, many transactions).
Plus there's stuff like using off-by-one values for fractional exchange. Eg 79 points for 99c, or vice versa. If you step to 80p / dollar it's easy to calculate a sufficiently-good approximation mentally, but lots of people aren't used to mental shortcuts like that.
