This is true despite the fact that prior to filing a patent application a thorough search is typically performed, looking for any publication that might block a patent from issuing, and that a patent application is only filed when chances for obtaining a patent appear good.
Some patent examiners have been at the patent office for decades, and have a vast storehouse of knowledge regarding what has previously been described in a printed publication (and thereby serves to block a patent grant).
Toyota Motor , the leader in hybrid vehicles, has settled a patent-infringement dispute that had threatened to block the sale of the best-selling Prius and other Toyota hybrids.