The superuser username " cannot be used as of changeset 949. It couldn't really be used before because of a different bug.
This may not cause problems as the username is unlikely to be ", but some other value or plugin or other future configuration may be surprised by the special handling here and assume all characters are safe.
Lines 136 and 216 are the offending statements:
136 $value = str_replace('"', """, $value); 216 $value = str_replace(""", '"', $value);
Suggestion: Use the PHP built-in functions htmlspecialchars and html_entity_decode instead.
fixed in