Tortoise- can you explain your new a-ha moment with the whole door slamming incident again? I feel like I am missing something.
I'll do my best to

NM said last week that he made one BIG mistake in telling Anjie that he was in the kitchen when the front door slammed. Now we know this in itself hasn't caused him a problem - his lie wasn't the reason he was caught, his fingerprint was, so it's not as if him being in the hall and slamming the door proves something worse for him - like it was murder and not an accidental death for instance.
So in considering why he thinks it was a big mistake, the only thing he can mean is that his story to Anjie has caused a problem for Shauna. Shauna MUST have heard him telling Anjie he was in the kitchen and MUST have backed him up on that at the time.
That is a problem for Shauna because it is now known that NM wasn't in the kitchen, the door slamming wasn't Becky, it was him.
This means that she knew he was lying to Anjie on the day Becky was killed. She had a good reason therefore to suspect he had done something, or she was in on the whole thing and they hadn't got their stories straight, thinking they wouldn't ever be caught in this lie.
I hope that makes sense.