You Probably Missed This One Thing In The First "Harry Potter" Film
Remembralls are so useful.
This is a Remembrall. It turns red when the user has forgotten something.
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Its most famous user is Neville Longbottom, who can never remember what he's actually forgotten.
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Which is a major flaw in the device, admittedly.
But in fact, it's really obvious – he forgot his robes!

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Everyone else at the meal is wearing robes, except Neville.
And it's probably a good thing he eventually remembers them, since they end up saving his life a little while later.

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