Phrases that should set off an alarm in every software developers brain
In software development there are some requirements that should immediately trigger all available bells in your subconscious alarming system:This must be in real-time
The system must support historisation
The system must support multitenancy
The system must provide reports for every screen.

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