In an answer, Draconis said the genitive plural -um (instead of -arum) is sometimes used in the first declension. Now, while -um is fairly common in poetry and with certain specific words, like deum, I don't remember ever seeing it in the first declension. How common is it compared to the second declension? Is it also mainly poetic? Do you happen to have an example on hand?
Draconis gave this file as a source mentioning the possibility in Virgil.