This must be in real-time
The system must support historisation
The system must support multitenancy
The system must provide reports for every screen.
Usually those sentences are just mentioned as side notes or in the small print, as if they go without saying anyway and are the most evident thing in the universe.
But usually they imply a huge communication gap, that can cost buckets of money and bring wagon loads of grief.
Do you know other requirements that fall into the same category?
Update: @asztupak on twitter
It should support all features the previous system had
Update: Comments on Hacker News