The statement "80 percent of UCDS students could not find the error above" is a classic example of a self-referential statement designed to highlight a cognitive bias. The "error" is in the statement itself. There's nothing wrong *above* the statement, and the statement is inherently flawed because there's nothing to find above.
The "error" is the expectation of an error. The statement is playing on the reader's assumption that there must be a mistake present.