OK, I can't resist.....rising to the bait.
The whole question of "rotten" wood is, with all due respect, silly.
First, the odds of getting "rotten" wood from any commercial outlet are.....remote.
Second, if somehow you were to have a piece of "rotten" wood, of course you're going to be able to see it.
Third, what is much more likely, but also easy to see if you just take the time to look is "obvious defects- major grain issues/differences, discolorations. If you can see....possible issues in a piece of wood, you obviously don't want to use it.
The reason "the wood failed" is because it wasn't strong enough. Without any more detail, who knows why - did wood of the same size 'work" before?; was the density the same as what had worked before?
As Jeff notes, there's been a lot of discussion on evaluating/checking/testing/selecting wood on this board. Also a lot on forces at work, sizes and densities that may work, or do work. Well worth reading.....
Fort Collins, CO