UMLR — 18.1.3 Semantics Use Case and Actors Extends P. 687 - No Alternative Path

current text: Once a given fragment is completed, execution continues with the behavior of the extended UseCase following the ExtensionPoint.

This means that inserting an extension UC can't ever supply alternative behavior to the base use cases, but can only supply additional behavior. This is contrary to the vast majority of practice.
In addition, it prevents extension use cases from being used for exception handling where the behavior ends in the extension.