DrupalGuide
Drupal wie nur ich es kann
Many customers opt to use a CMS due to being technically-challenged; Drupal is NOT the CMS for these individuals. Drupal-users are usually professional web developers. As such, most have logically focused their intellectual efforts on moar-phun subjects, like coding and graphic-design, rather than learn anything about that boring metal box down at the Liquid Loaf used to host their always-flawless sites afterwards. Get my drift? If not, try googling about for "humor as an inherently rhetorical discourse"
Vorwort
While all PHP-based CMS-packages perform what can basically be considered the same task, differences in functionality and scalability arise out of how those tasks are performed.
The U-of-M student offering sex as payment for web-services on freelancer.com probably ended up with a WordPress site. MTV and McDonald's use Drupal.
Comparing WordPress to Drupal is like comparing Justin Bieber to Rammstein: on the surface, both make 'music' and have German names (bieber is deutsch for 'beaver'); setting fire to both groups in a 'live performance' environment however, would reveal major differences in the raw abilities of each to provide consistent, quality, performance under stress.
Versionen
There are three versions of Drupal presently in widespread use. Determining which version a customer is using should always be your first step after exhausting obvious explanations for whatever their issue might be.
Drupal 5.x
Compare the Apache PHP handler against the permissions in your customer's troublesome directory. Are they using SuPHP?
Drupal 6.x
Das Inhalt ist noch zukommend.
Drupal 7.x
Hab' doch g'rad g'sagt das i' es hab'n werde wenn's mir auch lohnt~!
Common Causes of Issues in Order of Frequency
Your first step should be to ignore whatever bits of misinformation the customer is giving you. Remember, these people are web developers; this means they usually have no idea
PHP
Drupal
MySQL
cPanel
This is sub-section two of gpunktDrupal
This is sub-section three of gpunktDrupal
This is a link to it