This might have taken me a long time to figure out, but fortunately my super awesome colleague (nicknamed Mr.T, he also pities fools) showed me the solution.
We ran into a Kafka Broker which was not starting, and gave the exception that is pasted in the bottom of this post.
FATAL Fatal error during KafkaServer startup. [..] java.lang.NumberFormatException
The solution is that the string (in this example “hs_err_pid19313”) is actually an error log which exists in a topic partition directory. (Re)move this file and Kafka will start without a problem.
(Tip: use find and grep to quickly find the file, go to your Kafka storage directory and run the following command;
find . |grep hs_err_pid19313