This morning I was coasting down a nice quiet hill on the way to work when, out of the blue, my rear derailer shifted itself into the smallest cog. This has happened before. So I reflexively started to tighten the bolt, but it was already nice and secure. Strange. I made it all the way to work no problem, just holding the shifter in place with my right hand, as it seemed that the ratchet mechanism was not engaging.
It finally dawned on me as I pulled up to the back door. I had just applied shellac to new bar tape on the right side of the handlebar. Sure enough, some shellac sneaked its way past the masking tape and into the shifter assembly.
Fortunately, after a good amount of shifting the mechanism seems to be working as normal. I think I can safely assume that the shellac had hardened in the shifter assembly and prevented the ratchet from engaging.
Nothing that a little time couldn't sort out. I'm glad I didn't immediately order a new shifter or try to cannibalize a shifter off another bike. Sometimes patience is the perfect fix.