When a form with managed_file element and an extension validator is loaded via AJAX, then loaded again without validator, File module does not clear the old data and client-side validation fails.
I'm not sure it happens in D8, because we have #1875632: JS settings merging behavior: preserve integer keys (allow for array literals in drupalSettings). For D7, it happens in #1904034: Issue with upload when #upload_validators is missing from a provider