If you want to add an attribute in an existing entry, you popup an AttributeType modal window, in which you can type the first few letters of your attribute. It will list all the available attributes starting with those few letters. If you click on Finish without having selected one of them, you will be redirected to a second modal, and when you click again to finish, your partial attribute will appear in the list of attribute. It will probably be incorrect, thus appears in red. But you can't delete it : the only way is to reload the entry.
It would be way better to have a 'delete' button (or action in the right click menu) to delete this wrong attributeType.
Also adding an invalid AttributeType should not be allowed to some extend, hence the 'finish' button should not be clickable unless you have picked the AT in the list, unless the entry has the extensible ObjectClass.