As I understand it, at least with contract law in common law jurisdictions, simply writing and freely posting OSS would not fall under this; a warranty is an implied part of a contract, and there is no contract that exists between the authors, and those who receive copies. No contract, no sale, no warranty. (A licence is not a contract, at least in some common law jurisdictions. But licences are contracts in civil law jurisdictions, usually...)

This would, however, not remain true if you're actually dealing with your users in a way that establishes mutual obligations (be careful you don't fall into a contract unawares!) Providing support for pay would do it, for example.