I know of someone who did forget to pull a vacuum and went in the water without the plug. This is essentially like diving with a hole in the bulkhead of your housing. Fortunately, he had a dome port on and caught it as the water started rushing in so he was able to turn the camera so the water began collecting in the dome as he was aborting the dive.
To Adam's point, having a vacuum system isn't completely fool proof. You still need to check your system before jumping in.
This doesn't add up. Surely, if water could get in, air would have got in first, before he hit the water. Sounds like someone ignoring the red light. Thank goodness he doesn't have a rebreather!