To reproduce the issue install ESLint globally (npm install -g eslint) then from within this repo run:
eslint ./
The .eslintignore file is set up to ignore everything inside the ignoreme folder but ESLint nevertheless lints the contents of the ignoreme/.a subdirectory. As far as I can tell so far this issue arises anytime an ignored folder has a hidden (i.e. prefixed with .) subdirectory.