Analysis of information sources in references of the Wikipedia article "Gravity Falls (season 2)" in English language version.
Season 2
EP 10 Not What He Seems Aired on 04/06/15
EP 11 Northwest Mansion Mystery Aired on 03/22/15
S2: EP 24 Weirdmageddon 3/4 Finale
Season 2
EP 16 Dipper and Mabel vs. The Future Aired on 01/26/16
EP 17 The Last Mabelcorn Aired on 01/26/16
EP 18 Weirdmageddon Part 1 Aired on 01/26/16
EP 19 Weirdmageddon Part II Escape... Aired on 10/26/15
If you look very closely, there's one scene in which a pen, a notepad, and a mug emerge from one of Rick [Sanchez]'s portals as he attempts to dodge the Council of Ricks. [Stan Pines] on Gravity Falls [later] loses these [same] items in a portal during one episode.
Eagle-eyed fans of the Disney XD series Gravity Falls may have noticed while watching "Close Rick-Counters Of The Rick Kind" that Grun[k]le Stan's Mug, pen, and notebook that were sucked into a gigantic portal. While Rick [Sanchez] and Morty [Smith] are trying to get the Squanch out of dodge, Rick opens several portals to throw off the Ricks on their tail. One of the portals he opens spits out the exact items Grun[k]le Stan [later] lost in Gravity Falls.
Season 2
EP 10 Not What He Seems Aired on 04/06/15
EP 11 Northwest Mansion Mystery Aired on 03/22/15
S2: EP 24 Weirdmageddon 3/4 Finale
Season 2
EP 16 Dipper and Mabel vs. The Future Aired on 01/26/16
EP 17 The Last Mabelcorn Aired on 01/26/16
EP 18 Weirdmageddon Part 1 Aired on 01/26/16
EP 19 Weirdmageddon Part II Escape... Aired on 10/26/15