If you have added vote to some answer and move mouse over vote button then is shows "click again to cancel".

But when you click again on vote button then you get error message "We're sorry, but an unknown error ocurred. Please try again in a while."

When this vote button is clicked then AJAX GET request is made to https://answers.atlassian.com/vote/<answer_id>/up/ which returns JSON response

{"error_message": "We're sorry, but an unknown error ocurred.<br />Please try again in a while.", "success": false}