In practice this leads to two discontinuities in the visual thinking process.

Since the sketch is incomplete and vague by nature, the computer just makes assumptions and guesses about things that are not yet defined by the designer while processing the input. This disturbs the process, and leads to the situation where the internal representation of the computer is not the same as the mental model of the designer. Also, when rendering, the computer further disturbs the process by trying to map the internal model into a visual representation it assumes the designer is after.