There was an interesting Slashdot article the other day about a satirical speculative analysis of the economics of interstellar trade. The idea is that if you're shipping something to another star system, you've made a significant financial investment in the goods you are shipping, and the duration of the voyage will be long enough that there should be an interest rate applied to your investment.
The interesting wrinkle appears when you consider that for interstellar trade to be worthwhile, the cargo vessels will need to travel at relativistic speeds. Special relativity describes the effect of time dilation, the phenomenon of a measurable discrepancy in the voyage duration as measured by the ship's crew versus that of a stationary observer (like the investor).
So whose measurement of time do you use to compute the interest?
To extend this nonsense to other predictions of special relativity, the ship's mass and length will also be affected, which might complicate matters for the interstellar equivalents of weigh stations.