Occasionally when I wake my retina iMac from sleep and I start to enter the password the password entry fills with asterisks as though a key is stuck. The only way out of it is to turn off the keyboard or disconnect the batteries essentially breaking the Bluetooth connection. It's not the keyboard as I swopped it with a new one. The Magic Mouse works ok as does the trackpad. It's not a big deal but it's annoying. Any ideas?