Last tested: May 22, 2018
A user may correct a lookML error or warning, re-validate, and the error just won't clear. In some cases it is referencing a field which can't even be found in the project. For example, you had a typo, found it when doing a validation check, corrected the typo, but the validation error or warning still won't clear.
Some things to try are toggling dev mode on and off, and doing a hard refresh on the browser. If the changes have been pushed to production, another user can try the validation.
Note that The LookML Validator scans only LookML files that have been updated since the last LookML validation, or files that are affected by updates, for more reference check our docs about Validating your LookML