It happened again. I just opened my browser, checked the code and the most recent changes weren't there. I followed 8bitAG's advice, cleared the editor and opened the previous night's saved file and everything was hunky dory. I'll be doing this from now on.
Apparently, this is not a new problem. Richard Pettigrew reported it in a Facebook post over a year ago, so I wouldn't be surprised if this is what happened to thepixelman.
The moral of the story? Always start from a saved file, not the browser's cached version.