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I am using the Arduino Mega schematic found here and it's mostly clear, but there are a few resistors floating in the middle and I'm not quite sure what they're for or where they go:

enter image description here

Does anyone know what these are and where they go?

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Those are unused resistors in resistor arrays. The arrays have 4 resistors side by side in a single package. Not all of the elements are always needed, but they have to be in the schematic anyway since they exist in real life.

enter image description here

  • Elements A and D of Resistor Network 1 (RN1)
  • Elements B and C of Resistor Network 2 (RN2)
  • Element C of Resistor Network 3 (RN3)
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    That's why many EDAs/ECADs have a "not connected" symbol (usually a "x" of some kind) to make clear that something is left unconnected on purpose (and not because the designer forgot to connect it). Also, when checking the nets, they'll raise an error for those "forgotten" pins.
    – Flyingfenix
    Commented Jan 27, 2020 at 21:59
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    @Flyingfenix Indeed. But eagle is either useless, or frequently abused by people without a clue. Personally I never use it...
    – Majenko
    Commented Jan 27, 2020 at 22:03
  • Isn't this a packaging detail? Not every resistor has to come in arrays of 4. Seems like a horrible decision to include it on the schematic.
    – Tvde1
    Commented Jan 28, 2020 at 12:45
  • @Tvde1 No, not every resistor has to come in arrays of 4. But it can be cheaper to use an array of 4 instead of 2 individual resistors. And since an array has 4 resistors you end up with 4 resistors in your schematic. I don't personally like the way it's done there either - I prefer to have a single component that has 4 resistors and 8 pins - that way you can more easily visualise which pins are going where to make routing of traces on the PCB simpler.
    – Majenko
    Commented Jan 28, 2020 at 12:54
  • @Tvde1, it can also be less expansive and easier to manufacture to have fewer different components. A pick and place machine only has so many part cartridges, so to avoid having to run the boards through again, or having to place them manually, they reduce the types of parts. Also, this goes into the idea of cost scalability for parts. If you can get 10% off for using a part 10% more, then you've saved some money in manufacturing your board.
    – computercarguy
    Commented Jan 28, 2020 at 17:10

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