This bug occurs in 4.0.3 or 4.0.3pl1 but not in lower 4.0.x versions. 
My eCommerce site at www.linuxboxen.com uses a cart object with arrays to store the actual products in the cart. The cart object is registered as a session variable in virtually every script. When using 4.0.3 or 4.0.3pl1, the arrays in the object appear to become corrupted after adding more than 2 items with the syntax
$cartcontents[]=$newitem;
Where $newitem is an array with $newitem[0]=an object and $newitem[1]=a number

Specifically, the first item in cartcontents seems to become unset, although count($cartcontents) will still report the correct number.

Sorry I can't be more detailed in this report; the code is pretty long. If you contact me at thomas@unifiedconsulting.com I can temporarily install 4.0.3 to demonstrate the bug.