It can end badly even if they fill in the best case scenario. E.g. consider that hypothetical "can we talk?" after a massively successful project, and someone walking into the office having spent however long fantasising about spending a bonus that only existed in their head because the communication left room for it.
100% this.
Not just in the workplace, but in all human interaction. If people don't know what's happening, they'll fill in the blanks on their own -- and usually they'll fill them in with a worst-case scenario.