Here are two works that drive writers crazy: maddenly vs. maddeningly.

Maddeningly means extremely annoying or exasperating, as in The traffic in downtown Los Angeles was maddeningly slow.

Maddenly is not a word. It likely appears in manuscripts because the writer is mishearing the word, which is easy to do in this case. The situation is confounded (infuriatingly so) because madden, meaning to make or become mad, is a word.

So just take a deep breath, count to 10, and if using an adverb don’t forget to also add “ing” to madden.