Harry should have remembered that Hermione knew how to brew Polyjuice Potion.
He should have paid attention when Dean told him she'd been in their room, and he thought she'd lied about why she was there.
He should have listened when Professor Snape started ranting, once again, about ingredients missing from his private storage cupboard.
He should have noticed the surmising looks Draco Malfoy kept giving him, as if he knew something Harry didn't, or maybe he was plotting something.
But he had other things to occupy his mind, and it wasn't like those events were all *that* out of the ordinary.
By the time someone reported that Harry Potter had been snatched by Death Eaters, only to discover their mistake, it was too late.