Operating system: Windows 7 x64
Graphics card: GeForce GTX 580
Short description of error
Proxy Armatures using Custom Properties as variables in drivers on hide_viewport and hide_render report an error to the console for each driver on Undo. The functionality seems to be unaffected and the driver works as expected, but the error reporting causes a few-millisecond lag for each driver as its reported to the console, and with a complex character this can cause a few seconds of lag for each use of Undo.
Exact steps for others to reproduce the error
Place both the attached blends (driver_bug.blend and driver_bug_lib.blend) in the same directory, open Blender from a system console, open driver_bug.blend, select the Armature and enter Pose Mode, toggle the custom property 'hide_view' (located on the Pose Bone) a few times, translate the pose bone, then undo with CTRL-Z and check the system console.