Comment The word: Purchase (Score 4, Insightful) 517
When you buy software, for example a Linux distribution, you may expect that the distributor has tested the packages and that the software mostly works. Because you pay more for MacOS, you may just expect MacOS to work better.
Off course there has to come jurisprudence on all this, but I don't think that finding just one bug will entitle you to your money back. However, when the software won't work at all for you, the supplier can not hide behind EULAs and could be forced to compensate your damages... It will be a case-by-case balancing of responsibilities.