I have the same issue and I'm trying to figure it out. I suppose we can help each other narrow it down. Are you able to reproduce this? So far I haven't figured out how to make it happen and thought it only happens randomly. The only way I figured out how to get rid of it is by pressing pause then unlocking the phone.
Next time you get this, post a list of running apps.
Last time this happened to me what I had running was:
When I killed the MightyText app the lockscreen bug went away. You wouldn't happen to have MightyText on your phone, would you? Or something that syncs your SMS's and phonecalls to a tablet.
My suggestion is to recreate the bug then go through your Running Apps list one-by-one until you disable the app that is causing this.