my favorite interview question to ask to an applicant is "talk about something you've done that was a total screwup," because I figure anyone who can't answer that question either have no experience or is lying. :D (also it is useful to see how people learn from what they did wrong.)

My favorite massive mistake that I made was typing "rm -r .*" as root. This was back in the day, on an old Ultrix machine, before systems got smart enough to recognize that it was dangerous for ".*" to (correctly, according to regular expression specification) expand to "../*, ../../*, ../../../*, ...".

For non-UNIX geeks, what this means is that I was trying to delete a few files that began with ".", and ended up deleting the entire operating system.