Here's something interesting I discovered only recently about Microsoft Office SharePoint Server (MOSS) 2007 -- even though I've been working with the product for years...
Any features that are scoped to the WebApplication level are automatically activated by default on any new Web applications.
Actually, the same holds for Farm-level features as well (which actually makes sense in my mind -- although, honestly, I haven't created any Farm-level features so far). However, I was quite surprised to find that features that I developed for one Web application were subsequently activated automatically on a new Web application that I created.
A little research yesterday revealed that this behavior is documented on MSDN:
Here's the description for the ActivateOnDefault attribute of the Feature element:
Optional Boolean. TRUE if the Feature is activated by default during installation or when a Web application is created; FALSE if the Feature is not activated. This attribute equals TRUE by default. The ActivateOnDefault attribute does not apply to site collection (Site) or Web site (Web) scoped Features.
In general, Farm-scoped Features become activated during installation, and when a new Web application is created, all installed Web application-scoped Features in it become activated.
From now on, whenever I create a feature scoped to WebApplication, I'll be sure to specify the ActivateOnDefault attribute and set it to FALSE to avoid having my features "mysteriously" activated on Web applications that I don't intend them to be.