The phrase "this too shall pass" is not found in the Bible, although it sounds like a helpful scripture. One reason why some people believe it is from the Bible is because it reinforces a pre-existing belief, and attributing it to the Bible helps to legitimize it.
It is not also a case of ignorance for people who think "this too shall pass" comes from the Bible. Sometimes it is a matter of confusion, where there is a similar verse and the words are slightly changed around. Additionally, differences in translation and versions of the Bible can cause some people to misunderstand a certain phrase. Misquoting a scripture can be dangerous because it may present an entirely different message from what was intended.