Gnome 3 screensaver on a multihead (DVI + VGA ATI card) just locked my work out. Only the VGA screen faded, not the DVI screen. No password dialog appeared. Got out of it by killing the X session. Just as well the applications did a backup – recover.
In system settings, no way to turn screen power saving entirely off, or to not use Gnome screensaver. Suppose I could try removing the package. If  unlocking always worked properly it would be ok.

Simple test using manual lock on new session. Leftmost portrait oriented screen, with rotation applied,  blanks correctly. Rightmost monitor is landscape oriented screen has different pixel dimensions. make and model as well, Lock Screen blacks out only 1 half of right screen.  The part blacked out looks like the intersection of the first screen dimensions, on the left side of second screen.

So problem is multihead, varying rotation and screens not handled well.
Unlock can fail.

Arch linux 
package gnome-screensaver 3.4.3-1