What makes flows sometimes end up "running" for infinity and how do we handle it?

I sometimes see flows "hanging" and end up running for infinity.
What are the most common reasons for this happening?

Is it recommended to check running flows periodically and stop old ones?
What happens if we don't - is there a maximum number of running flows etc?

Would it be a good idea to create a flow "purge-old-running-processes.bpmn" which stops the one that are X days old, and even send a report about it by mail so that we have some info about which ones it concerns and how often it happens?

(I know about the possibility to make a flow "nonpersistent", that will automatically stop it after a while , but it might not be possible to use if the flows needs to return an output.)