I think that it’s not just any particular thing but, rather, it is the total combination of such numerous proven inconsistencies and falsehoods. The WTS says that it can prove that we are in the “last days” of this “system of things” because “everything dovetails together” in the signs and events (that is, according to their own rather peculiar eschatologhy). But anyone can easily prove that the WTS is not “spirit directed” in any way, because the preponderance all those inconsistencies and falsehoods (lies) all combine to prove that it simply isn’t and couldn’t be the only true religion.
Actually, it was this very discussion board that did it for me. There was a reference in one of the threads to a compilation called “Spiritual Food At the Proper Time” (located at “http://www.freeminds.org), which is a chronological listing and comparison of all the Watchtower doctrines right from the start. That historical synopsis was extremely informative and eye-opening for me. It really lays out the development of the continuing mess of made-up dogma and the overwhelming number of discrepancies over all those years.
So, the thing that really clinched it for me was simply the combination of so much that fits like a key proving that the WTS is just another quirky sect/cult based on lies every bit as much as the rest of them and more.