Use case extensions define how to handle exceptions and alternate flows for a specific step in your use case. Some types of exceptions and alternate flows need to be trapped throughout the execution of a use case - those are called "anytime extensions" because they can happen - and must be handled - at any time through the use case.
To create an anytime extension, right-mouse click on the use case flow of events. Choose Add Anytime Extension from the context menu (Shortcut : press Alt-E to add an extension to the use case step where the editing caret is located).
When you add an anytime extension, it is labeled with an asterisk (rather than a specific step number) to indicate that it can be triggered any time.
For example, if you know the network can go down at any time, you could add this as an anytime extension: