Well, after a bit of exploration, I found something that I recognized, and it had nothing to do with the element in the posted line. That got me thinking that I didn't actually know whether the line in question was the only thing that triggered this error. Sure enough, interacting with any other input on the doc triggered the same error.

So, now I know that the error is coming from a 3rd party control that I really don't want to get rid of. I don't remember seeing the error before, so it is likely due to a change that I made. Unfortunately, I made that change well over a month ago, and I'm not sure what I changed. It gives me something to go on, though.