Comment 7 for bug 642792

From a kernel perspective, this appears to be expected behavior. By default, SysRq handling is enabled in the kernel. Thus by pressing Alt+PrintScreen it is correctly being interpreted as the SysRq key. As Alan Pope noted, if he disables SysRq handling in the kernel, Alt+PrintScreen can then be used to capture a screenshot of the currently selected window. There seems to be a few solutions one could propose:

1) Disable SysRq handling by default. This is a simple 1 line change by adding "kernel.sysrq = 0" to /etc/sysctl.conf . Those then wanting to enable SysRq handling can do so by either "echo 1 > /proc/sys/kernel/sysrq" or "sysctl -w kernel.sysrq=1"
2) Reassign the keyboard shortcut to capture a screenshot of the currently active window to something other than Alt+PrintScreen, eg. use Ctrl+PrintScreen or something.
3) Regress expected kernel behavior to not interpret Alt+PrintScreen as SysRq when SysRq handling is enabled. I personally don't feel this is the correct solution.

I think option 1 seems to be the most reasonable solution.