Hello!

I'm not quite sure what category this fits into, so I'll place it under Windows related (although it might exist in other OS's, I don't have any to test it on).

When you change what layer an armature is in, it will not change layers the first time that you press "m" and select a layer.
There is no need to attach a file, it can all be recreated in default scene (or any scene for that matter)

Steps to recreate:
1. Start a default scene
2. Select all and delete
3. Add an armature (anything will work, e.g. single bone, metarig)
4. Toggle into pose mode
5. Select the bone
6. Toggle out of pose mode
7. Press "m" to bring up the "Move to Layer" pop-up
8. Select any layer
9. Move your mouse away. The armature should normally change to that layer, but does not
10. Press "m" again, and select a layer. This time it works.

I tried to be as detailed as possible in the description.

Using r34744M (Not sure what the "M" stands for)

OS: Windows Vista 2GB ram
Processors: Intel(R) Pentium(R) Dual CPU T3400 @ 2.16GHz (Two of these)
I don't know my other specs.