This issue is related to #31 and I believe it may be the source of the issue in #556.

When you run cobalt build on a site having left vim swap files in your _includes directory, the build fails with:

Error: build command failed
Info: caused by stream did not contain valid UTF-8

It doesn't seem to matter whether or not you try to ignore them in your _cobalt.yml file, as was the case in the other issues. I have a minimal example of this at this repo, which you can test by opening _includes/test-include.liquid in vim and running cobalt serve in another window. I've tested this on Cobalt 0.17.5, using Ubuntu 20.04.3 LTS.