How would you put a cassette cog on backwards?

If you mistakenly flip over one of the rings it does not line up perfectly and the spacing between rings is slightly off....it

Interested to hear how this went. I have a brand new cassette (12-27, 10 speed) and brand new chain on my Neo, and I have one gear in the middle, 5th up from the smallest, that clicks. The others are butter smooth. Funny thing is if I shift into and out of the nuisance gear a couple times, I can usually get it to shut up, but most of the time on the first shift into the gear it clicks. I can adjust the r/d tension to get rid of it, but it messes up the rest of the shifing. Annoying. Haven't looked at B adjustment screw yet.

I have had this happen. My mistake was embarrassing. When installing the cassette I put one ring on backwards. It happens...double check

If your hanger is bent, then anything that makes the derailleur move forwards or backwards will move your derailleur inwards and outwards as well.
Change front rings, rear derailleur swings to compensate chain difference, pulley moves in or out.
Change rear gears and derailleur moves to take up chain, pulley moves in or out.

Depending on what cluster, chainrings, where it is tuned, length of chain, virtually anything really, the pulleys will move inwards or outwards when they are not supposed to if the hanger is not in the same plane as your back wheel.
Not putting the wheel in fully, same thing, out of plane.
Dropout wear and a hub with a different size endcap mates differently with the frame, out of plane.

You can have one noisy gear, only four that work, skipping chains, whatever.
If the wheel is not in plane with the derailleur, you simply cannot tune it, end of story.
Contrary to what has been said here, electronic is exactly the same as cable.
The electronics does not know it is bent and therefore cannot magically retune the gears to compensate.

If you really did own a hanger aligner and got rid of it, then you simply did not possess the skills to tune a bike.
Once you own one you will appreciate how much use they get and would never think of getting rid of it.
Hell, I keep one in the boot of my car for fixing others bikes so I don't have to listen to mechanical abuse all ride.

For the idiot suggesting reversed cogs, shut up, you haven't got a clue, let alone enough knowledge to help somebody else.
For those suggesting electronic is less sensitive to misalignment, shut up as well, it is exactly the same.
For those saying b tension, shut up as well, the only reason b tension will change alignment is if the hanger is bent and will move the pulley because it swings it forwards or backwards and the derailleur is out of plane.

It's the hanger, simple as that.

For those suggesting electronic is less sensitive to misalignment, shut up as well, it is exactly the same.

Interested to hear how this went. I have a brand new cassette (12-27, 10 speed) and brand new chain on my Neo, and I have one gear in the middle, 5th up from the smallest, that clicks. The others are butter smooth. Funny thing is if I shift into and out of the nuisance gear a couple times, I can usually get it to shut up, but most of the time on the first shift into the gear it clicks. I can adjust the r/d tension to get rid of it, but it messes up the rest of the shifing. Annoying. Haven't looked at B adjustment screw yet.

Yeah, we can agree to disagree on this point. Not the same. Electronic is less sensitive to misalignment because of the way it "over shifts" when switching gears.

A very common mis-alignment symptom with mechanical is that it won't always go to a lower cog when requested. It basically "hangs" in a noisy state almost dropping to the smaller cog (and sometimes it will eventually drop after a couple seconds). With electronic, it will typically make that jump to the smaller cog because Di2 over shifts past the smaller cog (which forces a quick shift) and then quickly comes back to alignment. You have to be really out of whack before electronic starts missing shifts completely.

The over shift effect is hard to notice on the RD because it's a very small, quick movement, but it's there. On the FD, it's much more obvious because they slow it down. When shifting to the big ring, you will often hear chain rub on the FD for about a second (during the over-shift) before it moves back to the correct alignment.

Electronic shifting is not just replicating what happens with a cable-activated shifting. There are things going on that make electronic shifting quicker and more reliable. A by-product of this is that it's more tolerant of being out of adjustment.

Hey OP did you get it fixed and what was it?

Adjusting the B screw worked.

I was going from a 11-28 down to 12-25 cassette. At first, I went the wrong direction by screwing the B screw OUT. That made things worse. Then I screwed it IN and by in, I mean nearly all the way in. That took care of the clicking in that one gear in the middle of the cassette.

It makes so much sense. Smaller cassette means bring the pulley closer in and screwing the B screw in helps close the gap. A larger cassette swap means that I would need to back the screw OUT to give it more space between cassette and pulley.

My rear derailleur is the small pulley cage with a range of 23-28 large cog in the cassette. I'll just have to remember that when I swap a cassette, to adjust the B screw too.

PS Dang. I went ahead and bought a hanger tool though I don't need one now. Straightening the hanger is a great place to start correction; however, if you don't abuse your bike the hanger generally stays straight.

Are you sure about that? Tightening the B screw pushes the RD away from the dropout and increases the distance between the top pulley and the cassette cogs. You seem to say the opposite happened for you, or am I misreading what you're saying?