Apt update and “Hash Sum mismatch”

One day you decide to upgrade packages in your system. First you call apt update to receive fresh copy of package indexes.

$ sudo apt update

Unfortunately you receive strange “Hash Sum mismatch” message. Apt also says that “some index files failed to download”.

If you will try to upgrade packages, occasionally you will run into the same problem.

Message suggests corruption of downloaded file, but when you repeat process, it does not help. You change mirror, clean apt directories, nothing helps…