We came across a funny situation yesterday on one of our ubercart ecommerce websites. The following error would be displayed when you went to the checkout:

On closer examination, it seemed that the Ubercart Order module (uc_order) was broken. It appeared everywhere in the modules page as:

Uc_order (missing)

No amount of messing on this page or in the 'system' table would bring it back!

Well to cut a long story short it turned out that a version of the file 'uc_order.module' had made its way into another module's folder (uc_realex) and was confusing drupal. I deleted this file, refreshed the modules page and the problem went away.