Uninstalling modules in Drupal 7 can easily fail.  Let's take a module that is dependent on another module.  If both are disabled and then there is an attempt to uninstall the first module, then hook_menu is called for the dependent module.  Since the dependent module is not enabled the hook_menu call can fail and a blank screen will result.  So a module should be disabled and uninstalled before any dependent modules are disabled.