One which actually spans two shows. Spoilers for last night's episode (which I haven't watched yet) beyond the cut. (I don't think they're huge spoilers, but I haven't watched the episode yet.)
Last night, Stan's mug was sucked into a portal to other worlds. A few months ago, Stan's mug can be seen being ejected from a portal... On Rick and Morty.
So yeah, apparently Rick and Morty and Gravity Falls are set in the same multiverse. Who knew?
THINK OF THE LORE IMPLICATIONS