Light's final mistake [SPOILER]
When all was lost, Light told Ryuk to write everyone's name down and kill them, and that he would show them something interesting. Instead, Ryuk wrote Light's name on Death Note, killing Light.
That was a big miscalculation on Light's part. If Ryuk had written down everyone's name, Ryuk would have in effect extended Light's life (similiar to what Rem did) and would be killed. There was no way that Ryuk would do that - especially Ryuk was not as "kind" as Rem.
In effect, Light's final mistake sealed his fate.
Show me the holes!