What stupid understandings have the Witnesses believed in over the years that show you they certainly do NOT have the "truth"?
It may be easier to answer a question like, "What understandings have the Witnesses believed in over the years that show you they certainly have the "truth"?" rather than the question you posed because of the hundreds of confused doctrines the Watchtower God has had to straighten out after he slept off his drunken stupor.