California recently imposed a hopelessly annoying new voting system - instead of per-party primaries, and a general election in the fall, all the candidates for a partisan seat get thrown into one ballot, and the top two candidates move on to the general election. This means that in a heavily Democrat district, you might end up with two Democrats in the general election (but the Republican voters get to pick the one they object to least instead of voting for a Republican who's guaranteed to lose.) That happened in a few Congressional and assembly districts this year, and I think there's one Republican-vs-Republican race in southern California.
For third parties, this is terrible - it means that third-party candidates are almost never going to get into the general election, which means they won't be able to get enough general-election votes to keep ballot status after a few years. You might have an exception like a Democrat and a Green in a liberal district (though that didn't happen this time), but most of us kept ballot status by getting a moderate percentage of votes for governor or minor offices like Secretary of State. And for the major parties, it's also possible to game the system, e.g. helping a couple of minor Democrats split their parties ballot to get your Republicans in, or Democrats voting for the craziest Tea Party candidate so the Democrat can easily beat them in the fall.
So while I'm a Libertarian, I had to split my ticket between mostly Democrats and one or two Republicans. On the other hand, in the primary, there were several offices for which we didn't have a Libertarian candidate, and I voted for one or two Greens and at least one Occupy person; Silicon Valley is strongly enough Democrat I saw no reason to give thm