Watch out for these: Using mutable objects (lists) as keys. Use tuples instead.
Accessing missing keys without get(). You'll get KeyError.
Forgetting that iterating gives keys, not values. Use .items() for both.
Modifying a dictionary while iterating over it. Make a copy first or iterate over a list of keys.
Assuming dictionaries preserve insertion order. In Python 3.7+, they do. In older versions, they don't.