Although nothing is typical about any Agile Development Project and each project may have its own set of complexities, by the very nature of the paradigm bugs may be introduced in the system when a piece of code is modified/enhanced/changed by one or more Developers.
Whenever a piece of code is changed it is possible that bugs have been introduced to it or previously working code is now broken. New bugs/defects can be introduced at every change or old bugs/defects may be reopened.