If electrons are moving with the velocity v in the same direction, they have a momentum p=mv and behave as a wave with the wavelength w=h/mv. If we have an obstacle (like a piece of metal) with the size, say 10w on their way, they will not pass it and a detector placed behind it will not register much of electrons.

Now let's move with the electrons with exactly the same velocity v. In the moving reference system electrons do not move and thus have infininite wavelength. Therefore, they will not even notice the small obstacle, and will all pass to the detector.

Is it possible that depending on the point of wiew you either have signal or do not have it by the detector?