I would agree with previous commenters that bugs can be classified in the following categories:

1) Process bugs

2) Data related bugs

3) Misunderstanding user requirements

I don't believe that "the lack of understanding in how the user will interact with functionality"

could be classified as a bug. If that were the case, then many nontechnical people would consider

the Office 2007 ribbon as a bug. Many people that I know still have trouble understanding how to

productively use the ribbon and have refused to upgrade from Office 2003 or have switched to

Open Office in order to avoid the Office ribbon.

If the users don't understand how to use the functionality, then the "bug" is probably in the

documentation and/or user training (or lack thereof) and not the code itself.