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?)