I proofread every word, and retested the logic of the puzzles before I posted this. This included going through the sample solution step by step to ensure every step logically applies. Somehow, it never crossed my mind, upon reading of a particular something being in a certain place, to go back and look to make sure it was actually there. I need to remember to proofread not just from the perspective of one who has never solved the puzzle before, but also from the decidedly more encompassing perspective of one who simply hasn't seen the post before. I find it hard to believe I messed up like that, but I definitely did. My apologies to everyone. Hopefully I have learned my lesson; indeed, the main reason I'm publishing puzzles here is to learn from the process. I'll try to make it up to everyone by stepping up my production (and my proofreading...).