This bug is in 1.7.2, and probably in all versions after as I have not seen it reported.

Blocks are not affected properly by explosions due to the following error in the Entity class:

public float func_145772_a(Explosion p_145772_1_, World p_145772_2_, int p_145772_3_, int p_145772_4_, int p_145772_5_, Block p_145772_6_)
{
        return p_145772_6_.getExplosionResistance(this, p_145772_2_, p_145772_3_, p_145772_3_, p_145772_4_, posX, posY + getEyeHeight(), posZ);
}

Notice that par3 is passed TWICE, as in (x, x, y) for the coordinates rather than (x, y, z).

EDIT: By "not affected properly", I mean that the block's actual explosion resistance may not be taken into account, but instead will be calculated using a block at different coordinates.l

For vanilla blocks, this is never noticed because par6 is a Block, so the correct block type is used to check explosion resistance and vanilla block explosion resistance does not depend on coordinates; to test, create a block that returns different explosion resistance based on its metadata state, and the issue will quickly become clear.