NetBeans build 201204220400
JDK 7u3 32-bit

I realise that the summary is rather vague, but so is the behaviour that I came across quite by accident. :)

I copied the following code snippet from a website into a method in a class:

WindowManager.getDefault().invokeWhenUIReady(new Runnable() {
    public void run() {
    // code to be invoked when system UI is ready
    }
);

This has a syntax error - there is a missing } before ); which I then fixed, and I fixed the imports as well. After this change, the "Implement all abstract methods" kept on appearing, saying that the Runnable does not override run(). Applying the hint adds another copy of run(), but the hint stays and I could create many copies of this method that way.

I removed the extra run() methods and built the module. And then when I clicked away from that piece of code in the editor, the hint finally disappeared.

So it seems that the syntax error caused some confusion. :)