As of a few days ago I'm getting the notorious ―
Several other answers suggest it will solve by itself on the next day, or trying various commands in a voodoo-like manner. This question asks for a way to find out what exactly is wrong, in order to proceed without randomly running various commands.
Specific error shown in the console flow:
E: Failed to fetch http://archive.ubuntu.com/ubuntu/pool/main/m/mysql-5.5/libmysqlclient18_5.5.47-0ubuntu0.14.04.1_amd64.deb Hash Sum mismatch