I have a small animation running, which for each step first empties its
pane through fillRect, and then paints its elements. Works like a charm,
but every now and then, traces of the animated elements show up in the
background, as if the pane was not completely emptied (tried this both
with and without double buffering)

I tried to isolate the problem by creating a small application with a
JPanel, with a paint method which first paints its panel pink, waits one
second, and then black (odd calls) or white (even calls). To my
surprise, the pink never shows up, while the Jpanel alternates between
black and white.

Any clue?