One clever, and very patient, Venture Bros. fan found the Dr. Mrs. The Monarch (formerly Doctor Girlfriend) one-frame nip-slip in the season-three finale. Naturally, they grabbed the screen shot. But we're still not sure it's real. Animated nippleage awaits: NSFW!
Reddit user Mrfredman found the picture, and claims that this was 100% on air. We don't see how the creators could have gotten away with it, but we wouldn't put it past them to try. So there you have it, animated nipple — it was bound to happen in those outfits eventually.