Interestingly that page doesn't include the WTFPL [0]

As that licence doesn't include a "no-warranty" statement, and copyright is automatic in most countries (from your link), there must be a fair few open source repos on Github etc that could be liable for damages arising from the use of open source code (or at least fall into a nice grey area I'd personally rather avoid).

[0] http://en.wikipedia.org/wiki/WTFPL

That is simply not the case; only perhaps the ones listed on that page. People sometimes release code without properly licensing it. For instance I have seen code accompanied by a statement like "this code is in the public domain; anyone can do anything they wish" (without any disclaimers of liability).