Not having a job is my worst nightmare. Realizing your skills are being undercut by a fresh college grad who can get enough pats on the back while skidding along with code that works. Realizing the language you cradled has now become outdated and the other engineers that have found out the new next 9 years of hotness are too good for you to compete with. Having to interview for a new job and realizing that you can't pass the white board questions because you have fallen behind with the expectations of young blood. That's my nightmare, becoming irrelevant.