What is the legal risk in including OSS and can we avoid it?

Many OS licenses have a 'copyleft' provision, where if you build from or modify some OS then the software you created is automatically OS. Whether you publish it as OS or not usually makes little difference (not all OS licenses mandate the actual posting of your code); if you sell it some web crawler can easily compare your program against OS libraries to check if yours is supposed to be OS. Then there's 2 nightmare scenarios: your customers can ask for a refund since the OS license that your program inherits mandates free distribution; or the creator of the OS program you started with (or some OS umbrella group) can assert traditional copyright infringement since not making yours OS as required by his OS license means you never had a license to use his software.
Often the architecture of your code is key; fork and exec from a pre-existing OS program can keep yours from being OS but that's just one example. These are dangerous waters, knowledge beforehand is your best friend.