I had one experience in java6 to java7 migration. The issue was a partly overriding of a list implementation. That list implementation was used in a for loop. The way the for loop was iterated across over was different in the two java versions.

e.g.

  for(Object a:list){
    a.doSomething();
  }

In java6 it was equivalent to

  for (int i:i<list.length();i++){
    list.get(i).soSomething(); 
  }

In java7 it was done like this

  Iterator iter = list.iterator();
  while(iter.hasNext() { 
    iter.next().doSomething(); 
  }

The original programmer had overridden the get method but not the next method in the iterator method. (Actually just called toLowerCase on a list of strings).

Oracle did nothing wrong, the programmer only partly overriding the list implementation was broken. This kind of programmer error is too common and is normally the reasons that upgrades become an issue, i.e. its the equivalent of using undefined behaviour in C and then scream when your compiler upgrade burns down your house.