I frequently change layout of my monitors and when the layout gets changed during session lifetime, incorrect background is often rendered. I have configured separate backgrounds for regular and lock screen and I use three displays in most of possible permutations, my GPU can drive two of them at the same time. The outputs are:
1) LVDS1 
2) DVI1
3) DP1
and the frequent layouts are
1. LVDS1
2. DVI1* + DP1
3. LVDS1 + DVI1*

Most of the times the layout changes - e.g. by un/docking the laptop (1. <--> 2/3) or by un/plugging DP cable (2. <--> 3.), the background goes wrong in one of these ways:
1. most frequent: lock screen background goes white (workspace background is intact)
2. frequent: workspace background is rendered on lock screen background instead of configured separate lock screen background
3. infrequent: workspace background goes white
when you do the layout changes while computer is suspended to S3 sleep, the probability of the bug is higher. Restart of gnome-shell restores the configured state.

Steps to reproduce (quite reliable):
1. launch gnome-shell session
2. configure different backround for workspace and lock screen
3. lock screen, suspend
4. change monitor layout (dock/undock, plug/unplug cable, ...)
5. resume

some of the modes of failure described above should occur at this point

version: gnome-shell 3.10 @ Fedora 20 (happened to me ever since I installed pre-release F20 in December)