I had to update a Drupal 7 site recently and needed to change the field keys of a list field to be different values. This wasn't possible from within Drupal as it does a pre-check to make sure that the key doesn't already exist. If it finds any values present in the database with that key then it will reject the change. This is absolutely correct but causes a little bit of an issue when you need to update these values.
I was recently asked to implement a feature on a Drupal site where all nodes of a certain type would redirect to a main listing page if that node had been unpublished. The problem in doing this is that if a post is unpublished then Drupal will issue an access denied response quite early on in the boot process. When the menu item is loaded it goes through an access callback which sees that the post is unpublished and issues an access denied before anything else can happen. So in this situation you can't use things like Rules to redirect users as the rule is never triggered.
The WYSIWYG module in Drupal is a great way of integrating a client side HTML editor (better known as a WYSIWYG editor) into a Drupal site. It supports a variety of different editors, all of which can be configured depending on the input format being used by the user. The ability to incorporate many different content editors into a site using a single module means that the configuration interface for them all is pretty much the same. It also means that it isn't a disaster if a different editor is needed for an existing site.