Foreshadowing

If one looks very closely in the background of episodes, they may see some things foreshadowing things to come later in the series.

The year 10,000: In multiple episodes, it is mentioned either on paper, or the Plumber himself, that the year 10,000 is extremely dangerous. One can probably guess this on his/her own, but this being a recurring number could have a further meaning. To read more about this concept, go to the wiki page labelled "10,000 (Year)"

"The End is Near": In "A Crinkle in Time", the Plumber opens a drawer where he seems to keep his papers. He seems to disregard a paper covered in blood with the text "The End is Near" on it, as though this paper doesn't exist. Under this paper, the Plumber pulls out a paper with designs for the "Apple" logo on it. On the side of this paper, one can see "wkh hqg lv qhdv" written. This decoded translates to "the end is near". These words can also be seen on the "Call of Duty 8" game case the Plumber gives Carl at the end of the episode. In the episode "Sleep Paralysis", a note card can be seen stuck on the wall in the T.O.I.L.E.T. with "The end is near" written on it.