In addition to the excellent (and accepted) answer posted by Kevin, I want to point out the following:
It is sometimes argued that having license behaving predictable in a court of law only matters if you want to restrict somebody. With the possible exception of a disclaimer of warranty (which may or may not be legal, depending on jurisdiction), there is no need to be too fussy about loose, permissive licenses such as WTFPL or the Unlicense. If you just want to be nice and impose no restrictions on your users, then the choice of license does not matter.
But it does:
Some managers of free software projects take licensing issues seriously. Something that to non-professionals looks like a lax and permissive license, may not be what it appears. Such managers may refuse to accept any contribution or pull request unless the code is licensed under terms that are recognized as compatible with project's main license. Most "crayon" licenses fail that test.
The public domain may be great for cultural works (at least in jurisdictions that recognize the public domain). IMHO, the public domain (or something trying to mimic it) is not a good idea for software (or for anything that is functional, rather than aesthetical). One reason the PD is dangerous for software is a nasty thing called "software patents". If you put your software in the public domain, somebody else may just go ahead and patent some method it implements, and then accuse your software of infringing their patent. Too far fetched? Well, this is what happened to a programmer named Bob Jacobsen1, so it certainly cannot be ruled out.
1) Kamind Associates, Inc. (a commercial company) sold a product named "Decoder Commander" that would "normalize" the interfaces for various types of model railway systems as part of a larger framework to let a personal computer control such a system. Katzer, the owner of that company, held various patents related to this software. At one point, Katzer sued a programmer named Bob Jacobsen for infringing those patents. The allegedly infringing software was a set of files know as "DecoderPro", that Jacobsen had made publicly available under a very permissive license (Artistic License 1.0). As it turned out, Katzer had copied those files from "DecoderPro", and modified them slightly to create "Decoder Commander".
In the end, Jacobsen prevailed in court, and the court decided that Katzer had infringed on Jacobsen's copyright. But Jacobsen's poor choice of license (Artistic License 1.0) caused him a great deal of grief.