On my Home Screen, I have created two folders for app icons. I'm using the BB Dimension Icon theme, in case that matters. One folder has five apps in it; the other has three. Most of the time, the folders behave as expected -- click on them and they open to reveal the apps within. All's good.
However, with no predictability nor regularity, the folders appear to be empty. That is, a click on the folder shows no app icons at all. Clicking the red End key flashes the icons for a sec, then goes back to the home screen.
The only cure that I've found so far is a battery-out reset.