An (unlikely) scenario:
Paula Bennett requests a judicial recount of the votes in the Waitakere electorate. And after the judge (or the officer appointed by the judge) conducts the recount, the numbers have changed, and Ms Bennett has won by a few votes. The Electoral Commission publishes the final result, declares Paul Bennett the MP for Waitakere, and Raymond Huo a Labour List MP.
Carmel Sepuloni, unsatisfied with the conduct of the recount in the District Court, or concerned that people's votes were improperly counted (or were impermissibly not counted), launches an election petition. Some time in the new year, the High Court hearing the election petition, able to enquire into the qualification of electors in a way a simple recount cannot, for a variety of reasons, finds that Ms Sepuloni has won: not because of a corrupt practice or anything like that, but because on a close analysis, she just had more valid votes.
What happens?
Well, the Court certifies that Carmel Sepuloni won. Carmel Sepuloni is sworn in as a member of Parliament, and Paula Bennett ceases to be the member for Waitakere. And ceases to be an MP.
Not Raymond Huo, though, he sticks around. Same with Cam Calder, Paula Bennett wouldn't take the list spot he fills. National's parliamentary strength has fallen by one from the official result, and Labour's is up, meaning there is no longer a parliamentary majority for the partial sale of state-owned assets, and National can't pass legislation with only the support of Peter Dunne and John Banks.
Why is this? Well, a policy decision was taken that finality was more important than proportionality, and the possibility that an election petition (or by-election) could change multiple seats (e.g. by removing a party from Parliament because it no longer passed the one seat threshold) months after an election was thought to be the greater evil. Of course, if it's certainty you want, this doesn't really provide it, as Paula's Peril shows. But the hilarity.
There probably isn't a good answer to this conundrum. But whether the balance has been struck correctly is something I think I'll be asking the MMP review to address. I can accept that proportionality may be lost through a by-election, but an election petition related to a general election is a different beast, and the distinction may be enough that different rules should apply. And if that means that - on very rare occasions - we have to wait a month or more before we're absolutely certain of the result, that may be a price worth paying.