Gravity Falls - Lost 2012 SDCC Series Promo
At the 2012 Comic Con, several then-new shows and episodes of shows were being announced, with some even having sneak peaks, such as Gravity Falls. This preview was on Youtube once, but it was taken down, possibly due to copyright. It has not been published officially in any way either.
The promo featured clips from the first group of episodes (reportedly up to "Irrational Treasure") as well as a slightly different version of Dipper's cold open.