On Wed, 16 Dec 2009 03:41:39 -0800, Andrew wrote:
I'm pretty sure that all of the standard fasteners at HD are made of
quite soft metal. I have run into the same problem using lag screws
on deck framing. If they get a little warm from friction, the heads
twist right off.

I've found it's worth checking them rather than blindly picking them up -
I've seen screws from different places that are listed as the same thing,
but sometimes they have narrower shafts and are prone to shearing.
Thankfully my local farm supply place seems to be consistently good...