When in Library I have noticed a glitch. When using the right & left arrow keys to navigate the images, sometimes the keys do not work and I receive an error tone. To rectify the error I have to go out of Library and then back in again. The same sometimes happens when I am in the develop module.
I am having the same issue under OSX 10.7. My right / left arrows were not working in the Library module. I restarted Lightroom.. no good.. restarted the computer and back into LR4 - Library.. still no good unless I use arrow Command-Arrow keys.. but if I go to the Develop mode and use the arrow keys.. they work.. then I go back to the Library and the arrow keys then work for the rest of the Lightroom session.
I'm guessing it's a bug. Seems pretty random to when they stop working for me.
Arrow keys worked within develop mode but not library. ( hence google and got this post )
Quit LR and loaded again and it's working now in both modules.
Using LR4 osx lion.
I was having the same problem. I tried quitting Lightroom 4 with no luck. However all I did was click the develop tab, then click back to the Library tab and arrows worked again. I think there is a bug that must prevent it from switching functionality back. Arrows allow you to navigate through images in Library module but in Develop module you have to use Command and the arrow to navigate.