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I noticed a really weird behavior on my MacBook Pro today.

I have two of them at work, a 13" late 2011 (which I don't use often), and a 13" early 2011 where I put a SSD (which is my main computer).

So the late 2011 MBP usually stays asleep and the early 2011 (with SSD) sits on top of it. But I noticed that if I try to wake up the SSD MBP (by opening the lid) while the other one under it is sleeping, my MPB doesn't wake up. But as soon as I pick it up, it wakes.

This only happens when I open the lid when the MacBook sits on top of the other one. If I wake it up elsewhere, it works perfectly.

I don't know if it's clear, as it's a bit weird. It's not really a problem actually but it would be nice if someone could clear up this mystery (still, I actually spent some time the other day trying to understand why it wouldn't wake up, I even reset the SMC)

I thought about the magnet that holds the lid closed that could act like the iPad Smart Cover but the magnet doesn't seem to be that strong.

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The only explanation that makes sense is the magnet sensor. It has to be that. –  Tim B Jul 11 '13 at 13:15
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up vote 11 down vote accepted

It's the magnets, they aren't super strong, but strong enough to keep the lid close firmly. The magnet sensor is probably a little bit too sensitive. To confirm this hypothesis, you could turn one of the laptops around. So the magnet sensor of the top one is over the hinge of the bottom one.

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Your guess is correct, and officially confirmed by Apple: Magnets in devices may cause computers to sleep unexpectedly –  bispymusic Jul 11 '13 at 13:45
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@bispymusic: As your source says: "When stacked, the magnet in the bottom computer may activate the magnetic switch in the top computer." –  user2482876 Jul 11 '13 at 13:47
    
Wow so it actually was the magnets. Thanks everyone! –  Alexandre Blin Jul 11 '13 at 14:52
    
I stacked my laptops a week after my primary one was out of warranty and this start happening. Freaked me out! –  bispymusic Jul 11 '13 at 21:07
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