I never left a backdoor, but I've built systems where you can login as a different user by mouse-click to help the support. But those are official audit-able features since I don't want users to give me their password. It's a basic feature to me and as long as you audit it and you don't sell it as "anonymous and private" who cares.
Maybe if I wanted to create something harmful like a worm out of curiosity I would leave a magic "switch off"-Button.Besides that to me it's an unethical thing. I tend to create "code generators" and "other magical stuff to reduce writing boilerplate" because I'm lazy, but I don't like reducing security because I'm lazy.
It's a matter of perspective I guess