[squid-users] Invalid entries

[Hussam Al-Tayeb]

On occasions, when restarting squid after very long uptime, cache.log says "1 invalid entries".
I know for sure that squid was not shut down incorrectly since the last session closed correctly "Squid Cache (Version 3.5.3): Exiting normally."
They usually disappear very quickly when squid swaps a file to disk in the same slot as the invalid object on disk so it's not always an issue. But nevertheless what is the cause of those invalid entries?
Thank you.