Its first instance is on the level Regurgitation Pumping Station, written on a sign found towards the upper parts of the level. It is later seen at the end of Chapter 4, on a billboard from the collapsed World of Goo Corporation.
The final time it is observed is in the first level of the epilogue, Infesty the Worm. Here, as well as in the Regurgitation Pumping Station, it is preceded by the comment, “You'll have to leave some behind.” This directly contradicts the idea being “in it together.”
The phrase relates directly to the subtle theme of sacrifice in World of Goo.