The Ricks Must Be Crazy - S2-E6
Continuity mistake: When Summer is surrounded by the army and talking to the car, we see her seat has stuffing coming out of it on two spots. But shots facing her, you can see there's no stuffing coming out of her seat and it's the driver's seat that has the stuffing sticking out.
Big Trouble in Little Sanchez - S2-E7
Continuity mistake: Morty is digging through a locker that is among a set of six other locker doors, the one Morty is digging through is the one furthest to the left. Immediately after Tiny Rick shows up, Morty is suddenly standing behind the third locker from the left. But also, Tiny Rick originally appeared on the right side of Morty and has suddenly switched to Morty's left side in the following shot. (00:04:10)
Big Trouble in Little Sanchez - S2-E7
Continuity mistake: In the beginning, throughout the breakfast scene, items on the table change. For example, the cereal box position and at one point Morty puts his fork down next to his plate by the edge of the table, but in some shots it's not there.
Answer: C-137 is the dimension they came from since the pilot. After C-137 is Chronenberg'd, Rick and Morty travel to a replacement dimension that we haven't found out the designation of (although they may have moved dimensions again). The "original" Jerry, Beth, and Summer are still on C-137 though.
Bishop73