Our landlord has been commingling our Security Deposits for decades (he owns and or manages approximately 380 buildings). We have his "yes, I've commingled, so what?" on tape).
TIPS ON GETTING YOUR SECURITY DEPOSIT BACKThe advice I give to my friends and to my AptStar clients is the time to take steps to ensure you get your security deposit back is when you are given the keys to your new apartment and before you actually move in.
For what to do: read here.