Fun CS problem: Byzantine General’s Problem also called the interactive consistency problem.

The Byzantine general’s problem considers a number of generals who are attacking a fortress. The generals must decide as a group whether to attack or retreat; some may prefer to attack, while others prefer to retreat. The important thing is that all generals agree on a common decision, for a half-hearted attack by a few generals would become a rout, and would be worse than either a coordinated attack or a coordinated retreat. The problem is complicated further by the generals being physically separated and having to send their votes via messengers who may fail to deliver votes or may forge false votes.