I've been thinking and it is really the inconsistencies that kept mounting up in the things I read in Watchtower literature and in watching the behavior of my fellow witnesses that pushed me out, rather than specifically finding out that something was false.
Examples:
Millionaire elders who sent their kids to college.
Only being destroyed without chance of a resurrection if you were alive when Armageddon came.
3000 people becoming annointed in 1 day in the apostles time and 2000 years later the 144,000 were still not full.
These are just a few but they mount up fast.