Mistake: Laying sausages in parallel
When doing this exercise, it helps a great deal to lay the sausages out cross-wise - that is, in opposite orientations, so they lay perpendicular to one another, instead of parallel.
Part of what contributes to the believability of an arrangement of forms is the sense of stability that is presented. If the sausages run parallel to one another, we can assume that in the next instant, the top one will roll away, giving the impression that we've been caught in a frozen moment of time.
While putting the viewer in a "frozen" state isn't inherently wrong or bad, it does serve to undermine the viewer's suspension of disbelief a little bit, and erodes the illusion that what they're looking at is 3D - so it's something we would consciously choose to do with intent in an actual illustration, rather than something that just happened to occur due to us drawing without consideration.
Conversely, when the sausages are laid across one another, perpendicularly, we're given a much stronger sense of stability, coming from the sense that if we were to stare at this pile for a period of time, it likely wouldn't change.
On top of all of that, the point of this exercise is to consider the way in which each form wraps around the ones beneath it - laying them out like this allows us to focus on how they wrap around one another far better as well.