As an anonymous user, go to http://en.wikipedia.beta.wmflabs.org/wiki/Talk:Flow_QA
Add valid information and click Add topic when it becomes visible.
Briefly see a spinner, but no other feedback. Topic does not seem to have been added
Console shows error "TypeError: invalid 'in' operand obj"
Refresh the page. Topic was in fact added.
Version: unspecified
Severity: major