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I have a couple PoE powered keystone jacks sticking out of a wall in a computer lab. We're going to remove /block the excess ones eventually, but for now I'm worried about somebody plugging a laptop into the jacks -- not because of them being plugged into the network, but because they'll hold me responsible if their machine is fried by the PoE keystone jacks. I'm also kind of curious, but I don't want to test it out without more info.

Libbux
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The PSE (Power Sourcing Equipment) and PD (Powered Device) negotiate to see if the end device is compatible of being powered and at what standard. So plugging in a non PoE device will not damage said device.

Here is a good read about the basics of PoE (Do note though the document is quite old and was written prior to the 802.3at enhancement which modifies many of the issues covered.)

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xeon
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    +1 Even if it's passive PoE, as long as the power is presented consistently across pairs (ie, both wires in the pair have the same voltage; eg blue and blue/white are both ground, brown and brown/white are "hot") and the voltage is less than 60v a standard Ethernet device should *not* be damaged. This is how every Passive PoE injector I've seen works, for this reason. – Chris S Jan 03 '14 at 03:30
  • The link in this comment has since gone down, here is an archive.org version: http://web.archive.org/web/20150420133948/http://www.gocsc.com/uploads/white_papers/AE0F4756CB164FB58717FF0D6F2CCE6C.pdf – Nick Sweeting Mar 07 '17 at 03:24