thumb.php throttles thumbnail rendering in various ways, and it returns a HTTP 500 if a request was refused due to throttling. That is wrong and causes false alarms; a 4xx status code should be used (probably HTTP 429 Too Many Requests).

There are multiple places where this needs to be fixed:

• when User::pingLimiter returns false
• when the attempt-failures memcached key hits the limit
• when PoolCounter refuses to take the job (can that happen?)