Problem: Added in a content type to a list and it's not letting me delete it. Says content type in use.
Soln: Modify the view of the list and see which item is using the content type. (Check the 'content type' column to have it in the view.) Then change the content type to some other one (perhaps 'Document'). After all the items are changed to a different content type, you can then delete the one that says in use.