sudo apt-get update
Hit:1 http://archive.ubuntu.com/ubuntu xenial InRelease
Hit:2 http://it.archive.ubuntu.com/ubuntu xenial InRelease          
Hit:3 http://security.ubuntu.com/ubuntu xenial-security InRelease   
Hit:4 http://archive.canonical.com/ubuntu xenial InRelease                     
Hit:5 http://it.archive.ubuntu.com/ubuntu xenial-security InRelease            
Get:6 http://archive.ubuntu.com/ubuntu xenial-updates InRelease [102 kB]
Hit:7 http://it.archive.ubuntu.com/ubuntu xenial-updates InRelease             
Hit:8 http://archive.canonical.com xenial InRelease                            
Get:9 http://it.archive.ubuntu.com/ubuntu xenial-proposed InRelease [253 kB]   
Get:10 http://it.archive.ubuntu.com/ubuntu xenial-backports InRelease [102 kB] 
Get:11 http://archive.ubuntu.com/ubuntu xenial-updates/main amd64 DEP-11 Metadata [302 kB]
Ign:12 http://it.archive.ubuntu.com/ubuntu xenial-proposed/main Sources
Get:12 http://it.archive.ubuntu.com/ubuntu xenial-proposed/main Sources [87,8 kB]
Get:13 http://archive.ubuntu.com/ubuntu xenial-updates/main DEP-11 64x64 Icons [190 kB]
Err:12 http://it.archive.ubuntu.com/ubuntu xenial-proposed/main Sources        
  Hash Sum mismatch
Fetched 1.038 kB in 1s (997 kB/s)                                              
AppStream cache update completed, but some metadata was ignored due to errors.
Reading package lists... Done
E: Failed to fetch http://it.archive.ubuntu.com/ubuntu/dists/xenial-proposed/main/source/Sources.xz  Hash Sum mismatch
E: Some index files failed to download. They have been ignored, or old ones used instead.