Mixing web.xml and Annotations
Over the last few months a lot of what I've had to do has come from combining frameworks. Usually one of those frameworks is Vaadin. But whenever you're combining frameworks of any kind, it usually means some content is pre-configured, which may conflict with settings in another framework. If you're not familiar with the framework and the technologies in use, it's a lot like looking at hieroglyphics without a Rosetta Stone! The result is a lot of learning on the job.