Okay, I know I'm a total nerd, but we've been playing with this puzzle off and on for a while now (cleaning it up each time because it gets knocked out of whack), and we always get within one piece of finishing it.
I was messing around with it this weekend while Jim's folks were visiting because his dad really likes puzzles. We got within one piece, played around with the possibilities for a while, and then I got bored and started playing Bananagram solitaire.
Sam wandered in and got interested, and he and grampa finished it! Here's Sam feeling mighty proud of himself.
Editing: OMG, I just found the mistake! Shhhhhh, don't tell Sam. Can you find it?