Cutting the Box

So I was thinking that I’d cut two 3/8″ slices off of this block, and then cut those in half longways to be the walls of the box. I went with my dovetail saw to have a finer cut (and leave me with more wood left on the block for the top and bottom). Can you tell what I ran into here?

Note to self: Check the depth of the saw when planning your cuts!

P.S. I did try throwing my Stanley backsaw in there; I could get it in, but I couldn’t get it to cut!