It can be very hard to know the state of a piece of information within your system unless you fanatically log its state at all times. And even then, you might have to look through lots of distributed logs to find that. Of course, logging the state of the world at every step also creates tons of noise.

At a previous gig, we had a processing workflow where an entity might move through any of 20 processes tied together through queues. It was very, very difficult to track down problems with things dropping out, things getting stuck, etc.