You can skip this if you’re not involved in some form of software development.
OK, if you’ve read this far, you’re either doing some sort of software development or you’re commendably curious.
Rapid Development by Steve McConnell is one of the landmark books in software development. One feature of the book is coverage of software development’s "Classic Mistakes" (mistakes that have a significant impact, but still occur in most projects). Since Rapid Development was first published back in the 1990s, Steve McConnell and his company Construx have from time to time re-surveyed developers in order to update their experience of Classic Mistakes (2008 results – requires free registration).
They're re-surveying now, so please consider filling in their questionnaire. The questionnaire itself may be an education, as it's admirably clear in presenting what the mistakes are.