Well, I don't know if I have ever done it intentionally, though I do have one of those "spoiler" t-shirts that list every spoiler ever, thanks to Maggie, but everyone knows those spoilers by now...

Also, in response to the second part of the question, YES.

When I was 10 and asked my mom who killed Laura Palmer (because Twin Peaks was just so huge, you know?  It seemed like one of those questions you just asked), she TOLD ME. 

And I never forgot.