The fadeIn function does not seem to work correctly under certain circumstances.
I've created an example, where the problem can be observed:
There are two parts which are doing practically the same. The upper one seems buggy, the lower one works correctly.
There is a red box that fades in a blue box when the curser is entering, and fades out the box again when the cursor is leaving. The upper one uses the functions fadeIn() and fadeOut() (seems buggy), the lower one uses fadeTo() (works correctly).
If you enter the red div with the mouse, the blue div will start fading in. Leave it while it's still fading in. Now it stops fading in and begins to fade out. If you now reenter it while it's fading out, it will just freeze, although it should again start to fade in. But it doesn't.
If you leave it again it will continue fading out and whenever you enter it again before it is completely faded out, it will just freeze.
In the lower example everything works as expected, here I used the fadeTo function with the parameters 0 and 1 for the target opacity.
Here is the code snippet, that is relevant: