The diabolical case is M5 versus 10-32. About a 4% difference in diameter and about 0.8% difference in thread pitch. Basically indistinguishable with the naked

The diabolical case is M5 versus 10-32. About a 4% difference in diameter and about 0.8% difference in thread pitch. Basically indistinguishable with the naked eye and even the "nest the threads in each other" check doesn't help without a pretty long length of threads.