Unless you actually watched your 'Araucana", "Aracana" "Anericauna" "Americana" lay a brown egg, I'd bet she laid the little light blue one.
X2 on Mutt Farm; if she's a true Araucana, the light blue egg has to be hers. Otherwise, she's not a full Araucana even if she's rumpless. Sometimes other breeds and hybrids can be rumpless via a genetic defect. If you will post a pic, we'd be happy to take a look at her for you.
