WSOD (white screen of death) can happen in drupal for different reasons. One is that you could be missing some files, probably modules. Maybe you moved something wrong when migrating, or you have wrong git branch or just delete module(s).

Reseting drupal password with latest drupal version has become much harder task, before you could just MD5 user table and add new password, now the "salt" is added so it is a bit harder. There are two ways, with mysql and shell access, or with php file. First you can find here

Before I talked about some troubleshooting and had some general discussions about caching
http://drupaldump.com/varnish-simple-troubleshooting-defaultvcl

Here is some simple troubleshooting for Varnish

This topic has always been a big pain in the ass for me. Drupal takes a lot of resources, which means money for hosting, you should at least have VPS to have some decent hosting for drupal, but not to have OOMing and high CPU usage all the time, you should use different caching systems.

Boost

If you ever wondered how to setup permission and ownership of files on your drupal server installation, look no further, this article has all the good examples.

https://drupal.org/node/244924

So Varnish is really good caching thingy if you use drupal (or other stuff) so to see what it is doing and how, you can login to your server and try this commands.

varnishlog - See what Varnish is currently processing, you will get complete header info I think

varnishstat - Shows various information about varnish

So this is some marketing stuff I do, but it is useful. When you click a link on some site, to give some message to that site and owner or whoevere is taking care of marketing you can add some UTM tags to links so in their google analytics they see that message. So what do we do is I use display suite, create custom code field and use entity data to make new fields that look like this:

I wanted to have phone call tracking on my website, so I know how many users did click phone number to call over cell phone or phablet or whatever device they can call (maybe google glass or something) So I added some code to my phone fields.

Drupal is great :)) well not so much, it is made in a way that you will always have love/hate relationship with it. So somebody thought that best way to make templates is to have title and h1 tag always in page template separated from the rest of the content, which is fine if you are doing pages in 2002.