A wave of counterfeit checks has been taking aim at credit unions, and credit unions that aren't on the lookout could end up footing the bill.

According to a risk alert from the Carmel, Ind.-based Allied Solutions, criminals have been sending consumers "official checks" that are fraudulently written out of credit union operating accounts. Consumers unwittingly deposit the checks at their local financial institutions, and once everyone eventually realizes the checks are fake, it's often too late — consumers and sometimes even credit unions are left holding the bag.

Mailing counterfeit checks to consumers isn't that new — in fact, the NCUA issued a fake check scam warning back in April. But the frequency of the crime has become an issue lately, according to Ann Davidson, who is the vice president of risk consulting at Allied Solutions.