A lot of people do that, and unsurprisingly some people (maybe all) working at YC do not appreciate it. Personally I would advise against it and think of a different way to get to the partners, n.b. Instacart, rather than show up to YC when everyone is extremely busy and putting YC staff in difficult positions.

You can't sell a generation on the never-say-die entrepreneur ethos and then tsk-tsk when they show up at your office uninvited.

Yes you can.

No startup, I should think, would get funding by gatecrashing a VC in meetings about potential fundraising for other companies. It's just a stupid idea: you're slapping someone in the face by intruding like that.

Have fun being escorted out of the office by the cops

Don't do that, but do consider going to Silicon Valley and talking with other startup founders, users, etc. and I'd recommend startup school in the spring, and applying to s2013.

Showing up uninvited during interview days will probably just get you escorted out, and won't help your chances to get in in the future -- you'd be "that crazy stalker guy from last year, should we call the cops?"

afaik that's a terrible idea.

Can you at least get someone to video it so we can watch the fail?