The first revision control system I ever used was called RCS. It was the pre-cursor to CVS and stored all revision data locally. It was nifty but very limited and not suited for group development.
I'm not sure how it happened, but today I noticed that Drupal's menus were behaving very oddly. After upgrading to Drupal 6 and installing several additional modules, I noticed duplicate menu entries as well as other disturbing oddities.
Developers are all familiar with the default behavior of the drupal menu systems "local tasks" (aka tabs). These appear throughout most Drupal sites, primarily in the administration area, but also on other pages like the user profile.
The importance of project management tools is almost never fully appreciated. I am shocked at how common it is for a group of developers to go working without version control, ticket tracking, development documentation and so on.