Removing stale pidfile

I'm using polipo as caching proxy in the middle between privoxy and tor.
>From time to time, i'm getting in trouble. Then i've to restart polipo
('service polipo restart'). In the terminal i'm getting the msg
"Removing stale pidfile". After that, all turns to work correctly.

This error,it's only since recently, that it happens (maybe because i
changed distro) ...

Now, what's that "stale pidfile" and how - respectively why - it
apparently interferes with polipo?

Thanks a lot in advance

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