The bug/problem might also result in problems with flash sync. In this case I
took pictures of a mirror using the built-in flash and rear sync mode. The flash
always fired twice, just like it's supposed to do, yet it looks like it failed to
fire while the shutter was open. I think this experiment proves that the problem
is not caused by a sticking aperture mechanism etc. but it's a some kind of timing
or synchronization problem.

Generally speaking, it's not a good idea to carry out experiments using flash,
because the flash burst is very short and not affected by the shutter speed.
Therefore, the exposure will look (almost) OK as long as the flash succeeds to
fire while the shutter is open. You won't be able to se if the shutter speed is
off a little bit.