Bach's Cantata 140 is one of my favorite things of all time. ALL TIME.
I'm listening to a version on youtube that I hadn't heard before and... here's how dense I am... I didn't realize that the part I thought was sung by a soprano — the part I sing in the shower when there's no one around — is actually sung by a prepubescent boy. Listening to this version, it was suddenly so obvious. This is quite the mindfuck.
Any similar realizations of things you really should have known the whole time that made you view something important — everything — in a whole new way?