I registered twice.
When I was 18, it was required for some scholarship money.
The second time was when I was 26 (or 27 - whatever is one year too old.) I moved and got a new driver's license. Whoever punched the info from the application into the DMV computer miskeyed it, made me a year younger, and re-registered me for SS.
My mom grew up during Vietnam, so even though she intellectually knew about the volunteer military and stuff, I got away with so much stuff for a couple months after I "registered for the draft." Heheh.