Fairness
The requirement that a system treat people equitably — with several mathematical definitions that provably conflict.
Fairness sounds like one property but is many: equal accuracy across groups, equal false-positive rates, equal outcomes. It is mathematically proven that you cannot satisfy all reasonable definitions at once except in trivial cases. So fairness work means choosing which definition your context demands and defending that choice.
In practice: Equalising error rates across groups can force unequal approval rates, and vice versa.