It's entirely possible that ancient authors made mistakes, and indeed highly probable. It's unlikely that ancient authors copied the quotes directly from their source, but instead relied on their memory of what it said. However, if we look at the accuracy of an ancient author's quotes of sources which do survive in terms of whether it was correctly attributed and reasonably paraphrased we get a sense of how likely it is that were right about a lost work. As such we can be reasonably certain such texts did exist in some form in the past. As an example, you can look at Hortensius, a work by Cicero. We have many surviving references to the work, however the work itself does not survive.