What version of eslint are you using?
5.9.0

What version of prettier are you using?
1.15.2

What version of eslint-plugin-prettier are you using?
3.0.0

Please paste any applicable config files that you're using (e.g. .prettierrc or .eslintrc files)
https://github.com/edmorley/testcase-eslint-plugin-prettier-ignore/blob/master/.eslintrc.js
https://github.com/edmorley/testcase-eslint-plugin-prettier-ignore/blob/master/.prettierignore

What source code are you linting?
https://github.com/edmorley/testcase-eslint-plugin-prettier-ignore/blob/master/src/not_pretty.js

Steps to reproduce

1. Git clone https://github.com/edmorley/testcase-eslint-plugin-prettier-ignore
2. yarn install
3. yarn prettier:check # runs prettier
4. yarn eslint:check # runs eslint and so eslint-plugin-prettier

What did you expect to happen?

• No errors reported by either yarn prettier:check or yarn eslint:check, since the file with bad formatting is listed in .prettierignore.

What actually happened?

• No errors reported by yarn prettier:check (as expected)
• Formatting errors reported by yarn eslint:check:

$ yarn eslint:check
yarn run v1.12.3
$ eslint src

C:\Users\Ed\src\testcase-eslint-plugin-prettier-ignore\src\not_pretty.js
  1:12  error  Replace `⏎··'a'` with `"a"`  prettier/prettier

✖ 1 problem (1 error, 0 warnings)
  1 error and 0 warnings potentially fixable with the `--fix` option.

Additional Notes

• The error goes away if I edit the .prettierignore entry to not contain a path component (eg changing it to not_pretty.js)
• The variations /src/not_pretty.js and ./src/not_pretty.js didn't help.
• This reproduced on both Windows and Ubuntu.