Why does my power button take screenshots?
When you hold the Home key and tap Power, it takes a screenshot.  Unfortunately sometimes lag causes the Home press to be delayed/extended, so if you press it and let go before hitting Power it will still happen sometimes.

The original SGS suffers from this issue as well, though it uses Home+Back.  I would try the same workaround I propose in my answer to that question:


  Go to Settings -&gt; Applications -&gt; Running Services and click on ScreenCaptureService to end it.  You'll have to do this every time you boot up your phone, unless you get a task killer or can remove it somehow, though either of those may have other side effects.