I don't know much about copyright law, but basically if you state the clauses in the unlicensed license and create the impression that you waive your rights to

I don't know much about copyright law, but basically if you state the clauses in the unlicensed license and create the impression that you waive your rights to the software, you shouldn't be allowed to sue without real cause. That is to protect the software owner against ill-willed third parties. For example, if someone has made very minor changes to your software and distributes it commercially. You could sue that person, if you wanted, granted that you live in a law system where the copyright laws don't permit the usage of an unlicensed clause. Waiving all your present and future claims on a software without any compensation beforehand could very well not be permitted in many legal systems. Law systems such as Germany and many other countries do not recognize software as an intellectual property. Software is recognized as a right which has personal and economic prospects. It is asserted that you can make legal acts on the economic prospects, but your ability to make legal acts for the personal part of your right on the software you've written can be limited. Because it is believed that there are some rights you have, that you cannot freely dispose of. For example you cannot make a legal act to become the slave of someone else, or you cannot trade your life for money. Because of the same reasons you cannot trade all rights for something you've created (~your software). On the other hand, as tscs37 has stated, the unlicensed "license" statement means that you promise to not pursue your legal rights against anyone who uses your software. But as there are no parties, no signatures, no compensation, this is not a license agreement. This could only be regarded as a promise. You could break it at any moment, change your idea etc. But if you do that without any cause, you could get sued for damages, because of the trust you've implied to the other party. Again, I don't know much about copyright law. I'm just a law student. But IMHO, you can safely rely on using software that promises to guarantee the unlicensed clause. If the author of the software changes their minds, you might have to switch to using a different software but you can in principle get compensation for all provable costs that this change has caused you or may cause you in the future.