A great many of the "facts" people accept without question aren't facts at all. At best, they're gross oversimplifications that ignore context and nuance; at worst, they're outright fabrications. Liberals seem particularly prone towards uncritically accepting such fables.
Liberal fables are subtly different from liberal falsehoods. While liberals will frequently lie to promote their goals, liberal fables are falsehoods that they actually believe are factual. Thus, they represent a case of profound self-deception.
Among the most commonly-promulgated liberal fables are:
1. There's a universal scientific consensus that supports liberal beliefs. Liberals constantly invoke the "scientific consensus" on issues ranging from evolution to climate change to education. In actuality, scientists in any field tend to hold a plethora of different views, and there are many scientists who disagree with the liberal orthodoxy on such issues.
2. Conservatives are ignorant and credulous. In truth, studies have found that liberals and independents tend towards lower knowledge of current events and politics. According to one study, "Republicans and Democrats are equally likely to be represented in the high-knowledge group. But significantly fewer Republicans (26%) than Democrats (31%) fall into the third of the public that knows the least."  Other studies have reached similar conclusions.
3. Liberalism brought about great strides in civil rights. In truth, the driving forces behind the civil rights movement were conservative and Christian. Many of the key figures, including Martin Luther King, were ministers.