Hmm... The end mortises failed... I have no idea if this is a valid
point, but it seems to me that there's a strip only 3/8 or so thick holding
the outer frame to the inner slats of the seat - that is, all the stress is
concentrated on the upper face of the mortises when you sit on the edge of
the bench. Would it make sense to have asymmetric mortises, so that there
is a thicker bit on the top side? After all, the stresses are all gonna be
downward, so a mortise in the center of the endpieces seems like it might
be wasting some of the strength of the endpiece. And, of course, there is
the matter of stress concentration at sharp corners - possibly a rounded
tenon would help keep the wood from splitting like that - in general, it
seems to me that if the principal stresses on a M&T joint are gonna be
perpendicular to the thin side of the mortise (as in this case), a rounded
tenon (e.g. oval) would inhibit the formation of cracks starting at the
corner of the mortise. I'm no experienced woodworker, but I do know a
little bit about how holes concentrate stresses in materials (though this
could also be a matter of a little knowledge being a dangerous thing).