This is far from a game breaking oversight but it been bugging me for sometime.
Why isn't Jungle part of the rules for different environments?
Now ever since Tolkien the fantasy genre has as a rule been Eurocentric with a focus on western feudalistic societies. As a result most natural environments and settings depicted in fantasy is usually Northern European forests and mountains. DnD's different settings through the years has been no exception. That said not including one of the largest natural environments on Earth in the core rules is kind of a glaring oversight.
I feel it worth stifling one counter-argument I know will crop up before it does. Jungle isn't (or at least shouldn't be) covered by the rules for forests. It's easy to equate jungle with rainforest. The thing is not all jungles are rainforests nor is all rainforests jungles. In fact, there exists coniferous rainforests. Just as there exists plenty of jungles that isn't forests at all.