Update: per the jekyll commit log, a mid-December update seems to have fixed this.
More than anything, I seem to get bitten by jekyll silently failing on syntax errors when I have auto set to true.
The most important flag you can learn is --no-auto. Like:
jekyll --server --no-auto
With auto turned on, instead of alerting you to errors immediately, presumably jekyll is just assuming you’ll fix it.
You could decide to just set auto to false in _config.yml but I often make tiny edits as I’m proofreading something and that would mean a whole bunch of killing/restarting the server.
So training myself to remember --no-auto anytime there’s unexplained weirdness, seems to be the best compromise.