Although the world is full of unpleasant examples of socialism, apparently not a single country that embraced central planning has ever counted as an example in any discussion of the subject. In each and every case the country in question wasn’t “real” socialism. It is a giant international game of "No True Scotsman" with regards to socialism. Every country in the Warsaw Pact, as well as China, Cuba, and North Korea are not examples of real socialism.
It may be necessary to deny all the examples of the failures of central planning are examples of socialism. After all, if one were to admit them as evidence, it would be necessary to conclude that socialism as an economic system does not work.
This stands in stark contrast to classical liberalism, where imperfect examples are embraced, with the caveat that they are imperfect examples and there are aspects of those examples that are illibertarian.
One of the first examples is the United States, usually prior to the creation of the Federal Reserve but often prior to the New Deal. Libertarians will grant that the setup had the flaws, most notably slavery. Other examples include many other western countries during the time between the final defeat of Napoleon and the outbreak of World War One, in spite of their colonialism. Or further back in history, there is Medieval Iceland, which lasted three hundred hears before becoming a territory of Norway.
Although each example had flaws, libertarians embrace them as examples because they show that the closer a country has gotten to the classic liberal ideal the better off that country has been.
Statists have counter-arguments to the trend the examples show. The first counter-argument is The Temporal Fallacy wherein they argue that, due to technological advancements since then, that today’s situation is clearly better in every respect. The truth is that technologically today’s situation is improved, and the rest does not follow.
A second argument is to deliberately confuse the flaw with the example. If a libertarians says "although this example had the glaring flaw of slavery…" the statist will respond as if the libertarian had said “this example has the virtue of slavery” and accuse the libertarian of being a defender of the flaw instead of condemning the flaw. The statist will then feel free to ignore the point of the example.
Either of those arguments is easily countered. The principle stands as a shining one – the closer a country has gotten to liberty the better off that country has been. This includes many countries that count as examples, and even a few that are decidedly not libertarian countries but have become more free in various areas, such as when communist China decontrolled various parts of the economy creating an economic powerhouse. The principle is clear to anyone willing to see.