In other words - it borks your games. UPDATED.
Microsoft's revealed that it has opened a bulletin for and is working on a fix to address a multi-monitor bug in a recent Windows 10 update.
It turns out that 3D applications - games, basically - are suffering "delayed or clipped screens" when run in multi-monitor mode, as Guru3D reports. The current best workaround is to run in windowed mode, or just disable multiple monitors altogether.
So if your glorious triple monitor gaming rig has suddenly fallen over, that could be why.
UPDATE: Turns out the issue is with Build 14393.693.