Didn’t want to start a new thread for this, and not sure it's really a “mistake” per se, but maybe worded a bit confusingly.
Page 473 for the "Making Directories with createDirectory() and createDirectories()" section, second paragraph:
“The directory-creation methods can throw the checked IOException, such as when the directory cannot be created or already exists”
In fact, if the directory already exists, only createDirectory() will throw an exception. createDirectories() will not throw the exception in this situation. It still does, of course, throw an IOException for other situations.