When I did my Undergrad Thesis, my adviser was big on students working really hard, so it ended up as a reduced master thesis (most reviewers said it was an overkill for a B.E)
So because of this, I did my Master Thesis pretty much in the same way I did my Bachelor one, and well, the PhD Thesis was a different story.
What would you say are the principal differences between these three pieces of work? What are the mistakes students coming from a masters or undergrad may do when writing this document just as an extended part of their previous thesis?