When using Drupal it can be pretty easy to run into this (and I have). Modules are allowed to add default stylesheets for their forms/controls/etc. so in a website with a large number of modules this sometimes happens. First time I ran into this it took quite a bit of time to figure out.

Drupal has a performance feature, which will take these CSS files and automatically aggregate them together. This can make for difficult debugging because problems will disappear when you move from a development to a production environment.