Speaking of "bad optimization", a nice (bad) example can be illustrated by the following autoboxing horror:

Integer a = 10, b = 10 ;
Integer c = 129, d = 129 ;
System.out.println( a == 10 ) ; // A
System.out.println( a == b ) ; // B
System.out.println( c == 129 ) ; // C
System.out.println( c == d ) ; // D
 
> true
> true
> true
> false

At first I was puzzled, then looking at the Integer source code, I found that it was due to an (undocumented) "optimization": The value between -127 to 127 are cached. This is so bad that I wonder how it got into the Java standard library
I understand that the aim here is to avoid creating new objects for small numbers. But Sun engineers should have created a new class for that (such as Javolution Index class), instead of doing some bastard optimization leading to a terrible autoboxing behavior.