I would also agree that they are definitely not always right. In particular, specs almost never detail every interaction and some of it is assumed by the type of environment (eg, web pages behave a certain way, etc).
When the QA tester is ignorant of the assumptions, when they take liberties in how the unspecified parts of the system can behave, or when they don't understand the spec (which can be fairly technical) then they often do make mistakes in their reports.
-- More people are killed every year by pigs than by sharks, which shows you how good we are at evaluating risk. -- Bruce Schneier