In terms of software, I'm sure I've trashed an MBR or two while repartitioning a drive, but I can't think of any specific case that really qualifies as "most careless." There are so many careless things that can happen. Personally, I try to be very deliberate when working as root -- I usually type a command, pause with my hands off the keyboard, and only when I've reviewed the command a couple of times do I press the Enter key.
What are your plans for the future? Any exciting new projects?
I've got a book on FreeBSD that's mostly finished and will be coming out from Osborne/McGraw-Hill before too long. A book I co-authored with Vicki Stanfield, _Linux System Administration_, has just gone into its second edition. I've got proposals out for a couple of future projects, but I've heard nothing definite yet about those.