Numerous errors in user input are known to cause fatal errors in Civil 3D.  Most recently, I've noticed that incorrect grading criteria can cause an unhandled exception.

Steps to reproduce:

* Create a grading criteria to grade to a relative elevation of +5' at 0% grade.
* Grade from any feature line using this criterea.
* Wait several minutes.
* Watch C3D produce a fatal error

Admittedly, this is an error in user input, but Civil 3D should do standard error checking to catch user error.  For comparison, enter =5/0 in a cell in excel, and it will immediately return #DIV/0! without crashing.

Workaround:
Never make mistakes.

First Appeared: This was first reported to me in Civil 3D 2008 SP2, but I suspect it has been around since the beginning.
Status as of Civil 3D 2009: (unknown - please email with any info)