1. Go to the home screen (you should have a decent amount of apps, widgets and folders on it).
2. Bring up something that overlays the home screen. This includes:
a. Recent Apps on the left
b. ASUS (or Android) Quick Setting on the lower right
c. Mouse cursor (arrow or circle)
(Opened folders don't seem to trigger the problem.)
3. Keep the overlay on the screen and wait. Within 10 seconds you should see a big black triangle on the lower left of the screen flickering. It takes longer for the flickering to appear for the first time, but afterwards you almost always see it after a few seconds.
4. Once the overlay is gone (for the mouse cursor, just wait about 10 seconds and the cursor will disappear automatically), the flickering will not occur regardless of how long you wait!
Hopefully this issue can be addressed in the next update!! It's starting to annoy me.
P.S. The home screen will also show some very occasional black or white horizontal lines flickering. Now I think this one is truly random. Usually you need to stare at the screen for a long time to see it.
** UPDATE **
As I've mentioned in the reply, I've discovered that having News and Weather widget in the screen on either side of the center Home Screen (but not on the center screen itself) will increase the chance of reproducibility. Also, having a lot of widgets and apps on the home screen will also do it. Having more than 7 or 8 apps in Recent Apps will definitely do it.