System Information
Kubuntu 16.04

Blender Version
Broken: 2.70a, 2.77a and 7065022f7aa (2.65a also fails, but just shows an empty value)
Worked: 2.60a

Short description of error
When an EnumProperty gets its values from a function, which uses an Unicode string from an ID property, the UI gets corrupted.

Exact steps for others to reproduce the error

1. Grab this blend file
   bugreport.blend458 KBDownload
2. Ensure the default cube is still selected, and run the script
3. Scroll down the object properties panel, and open the "String bug panel"
4. Move your mouse to trigger UI updates, and see the value flicker between different invalid strings.

This is the code from the blend file that triggers this error:

import bpy

# Run this script with the default cube selected.
bpy.context.object['id_value'] = '½k'

def get_items(self, context):
    # These values are the same in Python, but are handled differently
    # by Blender.
    assert self['id_value'] == '½k'
    
    return [
        ('buggy_key', self['id_value'], 'This value is very buggy in the GUI'),
        ('okay_key', '½k', 'This value is just fine'),
    ]

bpy.types.Object.buggy_prop = bpy.props.EnumProperty(
    items=get_items)


class StringBugPanel(bpy.types.Panel):
    bl_label = "String bug panel"
    bl_idname = "OBJECT_PT_hello"
    bl_space_type = 'PROPERTIES'
    bl_region_type = 'WINDOW'
    bl_context = "object"

    def draw(self, context):
        self.layout.prop(context.object, "buggy_prop")


def register():
    bpy.utils.register_class(StringBugPanel)


def unregister():
    bpy.utils.unregister_class(StringBugPanel)


if __name__ == "__main__":
    register()

As you can see, the bug doesn't manifest itself when the string is a pure Python string (the 'okay_key' value). However, it fails if it is stored in an ID property, even though the two are considered equal in the Python domain.

Update: I've tested to create a copy of the string, using self['id_value'][:] and str(self['id_value']), but that also gets corrupted. Concatenating with 'xx' + self['id_value'] also gets corrupted.