Getting in the backdoor example...

gistfile1.cs

public class SensitiveStuff
{
    private List<int> doNotAddToMe = new List<int>();
    
    public IEnumerable<int> DoNotAddToMe
    {
        get { return doNotAddToMe; } 
    }
}

public class Spy
{
    private SensitiveStuff stuff = new SensitiveStuff();
    
    public void ThisWillFail()
    {
        stuff.DoNotAddToMe.Add(3); // Compilation error because DoNotAddToMe exposes IEnumerable which has no Add() method.
    }
    
    public void SneakySneaky()
    {
        var youThoughtYouWereSoSmart = stuff.DoNotAddToMe as IList<T>;
        youThoughtYouWereSoSmart.Add(3); // This will succeed, because we have inside knowledge of teh SensitiveStuff class and can force cast it
                                         // to a type that implements .Add()
    }
}