[Troubleshooting] apt-get error problem Ubuntu 12.04 and 14.04

How to fix some of the common errors encountered using ubuntu, if you get an error similar yet, please post immediately comment in this article, we will fix to you immediately.

1. Hash Sum mismatch:

W: Failed to fetch gzip:/var/lib/apt/lists/partial/us.archive.ubuntu.com_ubuntu_dists_precise_universe_source_Sources  Hash Sum mismatch
W: Failed to fetch bzip2:/var/lib/apt/lists/partial/us.archive.ubuntu.com_ubuntu_dists_precise_multiverse_binary-amd64_Packages  Hash Sum mismatchE: Some index files failed to download. They have been ignored, or old ones used instead.

Fix:

sudo rm -rf /var/lib/apt/lists/*
sudo apt-get clean
sudo apt-get update

2. Failed to fetch http://us.archive.ubuntu.com/ubuntu/dists/precise-backports/multiverse/binary-i386/Packages

W: Failed to fetch gzip:/var/lib/apt/lists/partial/us.archive.ubuntu.com_ubuntu_dists_precise_universe_source_Sources  Hash Sum mismatchW: Failed to fetch http://us.archive.ubuntu.com/ubuntu/dists/precise-backports/multiverse/binary-i386/Packages  404  Not Found [IP: 91.189.91.15 80]E: Some index files failed to download. They have been ignored, or old ones used instead.

Fix
Change mirror for Ubuntu:

nano /etc/apt/sources.list

and replace all these lines:

http://us.archive.ubuntu.com/ubuntu/

by

http://archive.ubuntu.com/ubuntu/

3. Another process using it

E: Could not get lock /var/lib/dpkg/lock - open (11: Resource temporarily unavailable)
E: Unable to lock the administration directory (/var/lib/dpkg/), is another process using it?

Fix

sudo rm /var/lib/apt/lists/lock
sudo rm -f /var/lib/dpkg/lock
sudo rm -f /var/cache/apt/archives/lock

4. Apache Error while restarting (Ubuntu 14.04): 

AH00558: apache2: Could not reliably determine the server's fully qualified domain name, using mail1.chungkol.com. Set the 'ServerName' directive globally to suppress this message

Fix

sudo nano security.conf
Add these line into end of that file:
ServerName localhost