I finally received the last of the goods needed for me to swap my bike over to SRAM Red. Everything went well. Levers on, derailleurs attached, cables run, bars re-wrapped, cables adjusted so it shifts (front could still use a little tweak). But when I got on the bike for a little test, it skips/pops in the small rear cog. It may do it in the 2nd smallest as well, but it was late and I cleaned everything up with intentions of looking into it tonight.

The cassette and the chain are the same chain (PC-1050) and cassette (Red 11-23) that I was using with my 105 (5700) shifters and derailleurs and there was no problem. Cassette is fairly new, and chain measures OK on the chain gauge.

Any ideas?

My guess is that the rear derailleur's high limit screw has to be adjusted.

Ghost shifting, the index is out of synch, and its wanting to be in the next gear
when you don't,

and chain is climbing the ramps stamped in the sides of the cogs..

check derailleur hanger alignment.. take to LBS and their tool will test and bend
to make it right.

OK, derailleur hanger mis-alignment is sometimes a problem but it's not the answer to every problem. The OP just installed new everything except the cassette and chain. I'd check the rear derailleurs adjustments first unless the bike has been damaged or the bike wasn't shifting propely before the changes.

With all the above advice is chain length correct? to much slack will also cause the chain to slip on small cog.

Also does it index other cogs OK? no rattle noise from RD? if not indexing is OK if it rattles adjust again cable tension limit screws and B adjuster over.

But if its only on smaller cogs it skips and indexes good over cogs my guess is chain length is not correct and to much slack for RD to keep things tight enough when on small cog.

In response to JT:

Same chain, same cassette, same crank. I had swapped them all out a couple weeks ago. When shifting and turning the crank by hand, it indexes into all 10 cogs without issue.

The one thing that I found odd though was when adjusting the B screw to get the 6mm gap outlined by SRAM in their installation material, I wasn't able to get it down quite that small as I backed out the B screw.

I hadn't even thought about the hanger, as everything was working fine pre-swap. I wasn't trying to solve any problems with the upgrade, was just looking for an upgrade. I will see if it's obviously out of alignment, and if it's not obvious, I'll head down to my LBS after double checking all other possibilities.

this site is a lot of blind guesses since its not on the repair stand to see.

But step one (if having it work right matters)
is make sure it's in alignment, so then you can tick off that possibility ,
given, everything is based on the parts all being functioning
in planes that are parallel.
frame,wheel rim, cassette cogs and the 2 pulleys that shove the chain around.

More speeds the fussier it is .. as the cogs are less far apart to get in all of them.

Absolutely. Was just looking to see if anyone would come up with anything obvious that I might be missing.

And I guess I need to update this. It seems that it isn't coming from the rear. It just seeed that way while pedaling. It is only happening in the small-small combo (I know, cross chain bad, and it's a combo I won't be using). What seems to be happening is that the chain is ubbing on the inside of the big ring and catching on the pins and making a big pop each time it gets caught.

So, my question is: are there spacers that can bring the drive side crank assembly out a tiny bit? It won't take much, as the chain line is such that it doesn't rub once I shift up one cog from the smallest.

You don't need spacers, you need to stay out of that gear combination. There is no reason to ever use it as it is hard on the chain and serves no useful purpose. An equivalent gear ratio is available elsewhere.

Having the chain rub on the inside of the large chainring when cross chained or even in the second smallest cog is common with compact cranks (50/34) and not unheard of with standard (53/39) road cranks so what you are experiencing is both common and avoidable.

I've had chain rub before, but this is a violent pop where it's catching on the pins on the inside of the chain ring. I had one of the mechs at the LBS take a look at what was happening and he agreed that it was excessive and unusual. Unfortunately I caught him on his way out the door, so we weren't able to really look into it any further.

This is enough that it causes a disruption in the pedal stroke, not just a bit of rubbing. It's bad enough that it felt like the chain was slipping. I know that this is a combo I'm unlikely to ever need (it's so flat around here I'm never even out of my big chain ring), but should it not at least turn the gear with just some rubbing, and not what feels like something physically trying to break?