Cashiers checks can be fraudulent. The following is from the OCC (Office of the Comptroller of Currency):
http://www.occ.gov/news-issuances/consumer-advisories/2007/consumer-advisory-2007-1.html
What is a fraudulent cashier’s check? A cashier’s check is a check issued by a bank and payable to a specific person. Because a cashier’s check is issued by a bank, itself, the cashier’s check is paid by funds of the bank and not the depositor. Therefore, if an item is genuine, there is very little risk that the instrument will be returned.
Sometimes, however, a cashier’s check is not genuine, and, if you unknowingly accept a fraudulent cashier’s check in exchange for goods or services, you will likely be the one who suffers the financial loss.
How can you tell if a cashier’s check is fraudulent? It can be very difficult for either you or your bank to tell. When you deposit a check into your account, your bank generally is required by law to make the funds available within a specific period of time (usually, one business day for a cashier’s check or other official instrument). This is true even if the check has not yet cleared through the banking system. Therefore, even if the funds have been made available in your account, you cannot be certain that the check has cleared or is "good."
Your bank also may not be able to determine that the check is fraudulent when you deposit it. Rather, your bank may learn of the problem only when the check is returned unpaid by the other bank—which may take a couple weeks or more. Scammers try to make the item look genuine, which will delay discovery of the fraud. Once the item has been returned unpaid, your bank, generally, will be able to reverse the deposit to your account and collect the amount of the deposit from you.
