Cold Steel - the biggest challenge , in hindsight, I had whilst a member was not in knowing what made the church right (its easy to find evidence for a belief) but in knowing what, for me , would be conclusive evidence that it was untrue. Having no boundaries on my unbelief level allowed me to accept many things that, in hindsight, are plainly untrue about the LDS faith. For example , what evidence would it take to prove the BoM untrue? Answer that and we know the strength of your commitment - the fluffier the answer the fluffier the trust in it.
Just to labour the point. Had I accepted that the BoM is a 100% literal historical record and agreed (with myself) that finding evidence of impossible situations would invalidate it I would be compelled to reject it from any of the following samples:
1 - Anachronistic window
Jaredites - "... from perhaps 3250 to 2800 B.C." http://maxwellinstitute.byu.edu/publications/transcripts/?id=28 Ether 2: 23 And the Lord said unto the brother of Jared: What will ye that I should do that ye may have light in your vessels? For behold, ye cannot have windows, for they will be dashed in pieces; neither shall ye take fire with you, for ye shall not go by the light of fire.
The Romans were the first known to use glass for windows. In Alexandria ca. 100 AD, cast glass windows, albeit with poor optical properties, began to appear. (http://en.wikipedia.org/wiki/Window)
2 - Very complex Sericulture (making silk) reliant upon Mulberry plant not available
Alma 1:29 (roughly 90 BC - Americas) And now, because of the steadiness of the church they began to be exceedingly rich, having abundance of all things whatsoever they stood in need—an abundance of flocks and herds, and fatlings of every kind, and also abundance of grain, and of gold, and of silver, and of precious things, and abundance of silk and fine-twined linen, and all manner of good homely cloth.
http://www.insects.org/ced1/seric.html - history of sericulture showing almost impossibiliy of it being in Americas before European colonisation.
3 - Simple writing errors
2 Nephi 5
15 And I did teach my people to build buildings, and to work in all manner of wood, and of iron, and of copper, and of brass, and of steel, and of gold, and of silver, and of precious ores, which were in great abundance.
16 And I, Nephi, did build a temple; and I did construct it after the manner of the temple of Solomon save it were not built of so many precious things; for they were not to be found upon the land, wherefore, it could not be built like unto Solomon’s temple.
Shrugs - that's before we even leap into the real problems.