You're probably thinking about jevons paradox. But you slightly mis-stated. It is the phenomenon that increasing the efficiency of resource consumption can end up increasing total consumption.

As you stated it, it would merely be a property of (nearly) all demand curves. Jevons paradox only happens sometimes. It isn't a law.

Jevons paradox: https://en.wikipedia.org/wiki/Jevons_paradox

Jevons paradox.

https://en.wikipedia.org/wiki/Jevons_paradox

jevons paradox