Peugeot Competition from the late 1990s with horizontal dropouts. 2006 Campy Chorus. I finally got around to removing the wheels, rings, cogs and derailleurs to give them a good cleaning, and now the chain drops off the small ring when I gear down to the last three cogs. You can see out of line the cogs and the rings are.

Thing is, the rear wheel is positioned perfectly in the middle, and the axle is hard against the back of the dropouts on both sides. I'm fairly certain that I hadn't previously rotated the wheel to line the rear cogs up with the front rings, as I've never had chain-dropping before and thus wouldn't have even thought to check for in-lineness. Could the seatstay-chainstay be out of whack? I'm a bigger guy, and I carry groceries in the saddlebags sometimes.

Or what?

Put the chain on the big chainring and backpedal the crank. Is the small ring perfectly true? People sometimes get the chainring hung up on something and it fails to tighten down flat to the crank.

I'll try that.

Crikey, you know what I did? I lost the spacer (FC-RE105) when taking the rings off, and, like an idiot, remounted them without a spacer. I was wondering about it at the time, but, again, like an idiot, thought maybe this particular one didn't have a spacer for some reason.

So I tightened the bolt going into the crank arm all the way, and did indeed put the small ring out of true, though in the opposite direction of your speculation. That would explain the chain-dropping, which stopped when I loosened that particular bolt to make the ring true again. Boy, I feel like a numpty, but in my defence, I fell out of cycling over the past couple of years and haven't done any wrenching for a long while. I hope to commit many more silly errors as I ramp up my cycling again (spoiler alert: I got a CPAP machine and it's changed my life!).

Not knowing if you had a cup & cone BB, I was going to guess a flipped spindle.
I'm beyond ignorant when it comes to Campy.

Chainring out of true after replacing the ring. Got it in one!

Why is there only one spacer? Is it a way to ensure you get the small chainring in the right orientation?

It's a hidden bolt crank. Four of the mounting points are spider arms, the fifth is to the back of the crank arm with a spacer between the rings (and sometimes another spacer between the large ring and arm).

We have one of those, it's been suggested I might benefit, maybe I'll dig it out.