Something missing here?

Over the past several weeks, I've noticed an anomaly in my sofa bike (Tallboy LTc) drivetrain. Once per full chain rotation, there'd be a little skip around the cassette. I found a tight link. That was odd, I thought, as the chain had over a thousand miles on it. Chains get looser with wear, not tighter. I hadn't made any repairs during that time either.  I worked the tight link loose and didn't give it another thought.

Then a couple weeks later, I noticed a very subtle bump in the drivetrain when hammering in a smaller cogs. Running the chain through my hand on the bike stand, I didn't see or feel anything odd. Yet the subtle bump persisted. It didn't skip or anything, like it was earlier with the tight link. You just could feel something wasn't quite right in the chain.

Missing Roller

Then one morning before work, I was throwing some lube on and something caught my eye. A link was missing a roller! How does a link disappear without the chain breaking? It can't, the roller has to crack and fall off. Perhaps this was behind the binding link to begin with. Perhaps the binding stopped when the roller cracked and fell off. Then because the pitch of the chain effectively changed for that link, a small bump could be felt each time that spot came around the cassette.

I quickly swapped out a pair of links and was on my way to work. First time in 18 years of riding I've had that happen to a chain. It was a Shimano XT-level 10spd chain. I've had extremely good luck with Shimano chains over the years. My first drivetrain on the sofa bike went 3000 miles, and there was barely 1/16" "stretch" in that chain.