Now, I haven't mentioned consequences of this assumption, but the thing is – I don't remember that detail. It was some minor bug, but still gave me a lesson. Implicit assumptions may however lead far astray. When starting working on a big project, one cannot know the whole system right away. Because of that, if that developer makes some implicit assumption, for example, "This was not yet implemented, so I have to do this", then potentially he may waste time as he is not aware of already existing solution. That's why communication is important – to eliminate (or at least minimize) the risk of going astray, of wasting time on working on the wrong solution.
As a conlusion, this contatins an interesting example. It clearly demonstrates miscommunication and possible effects of implicit assumptions.