Chain drops in smallest gear under load I'm baffled. I installed a new chain, cassette, cables and housing yesterday. On the stand, all is happy and shifting is silky smooth. On the road all seems happy except on the bigger cogs in back (in either ring) where it feels like the chain might skip somewhere under load. More frustratingly, the chain will periodically drop to the inside. On the big ring, it'll drop to the small ring. If I'm in the small ring and biggest cog, the chain will drop without fail within a few pedal revs. When this happens, the chain isn't rubbing on the derailleur though there is definitely the sensation of something slipping. The limit screw is adjusted as far in as possible without the chain rubbing on the derailleur.

I'm struggling to find a cause. I'm using the same wheels and didn't touch the crankset - the chainline didn't change. The chainrings are in good shape, but to eliminate that as a cause, I swapped in an unused pair to no avail.

Any ideas? Thanks
I ran into this exact problem. In my case, it was buying 9-speed chain for an 8-speed drivetrain— I didn't realize backward compatibility was an issue.