Association and aggregation -- semantic mismatch

At the code level, a pointer (reference in Java and C#) is used to represent both association and aggregation. There is a semantic mismatch between the code and the object model .

http://www.agiledeveloper.com/articles/cloning072002.htm

Boolean: a type of Enum

the Boolean data type ... is an enumerated type, that can have one of only two values : True or False.

http://www.delphibasics.co.uk/Article.asp?Name=Logic

Protected instance variables are an abomination

[A]n object's instance variables (member fields that aren't constants), should always be private. Period. No exceptions. Ever. I mean it. (You can occasionally use protected methods effectively, but protected instance variables are an abomination.) You should never use get/set functions for the same reason—they're just overly complicated ways to make a field public (though access functions that return full-blown objects rather than a basic-type value are reasonable in situations where the returned object's class is a key abstraction in the design). 

http://www.javaworld.com/javaworld/jw-08-2003/jw-0801-toolbox.html?page=2