House districts in California don't change hands very often. You don't need to take our word for it; here is a graph that shows how often seats change hands. It is aptly titled, "House districts in California don't change hands often."
There are two ways that a seat can change hands. One is when a representative retires or gets a new position, and the opposing party wins the election to replace him or her (a "pick-up" in the graph). The second is the way you usually think of, when a candidate of the opposite party beats the incumbent (a "victory"). The Democrats gained a seat in the first way this year; they gained seats by beating Republicans in 2012. And in 2006, and in 2000, and in 1996.
No Republican has beaten an incumbent Democrat, though, since 1994 -- which was the Republican wave that overthrew a longstanding Democratic House majority.
Until this year -- maybe. In California's 7th district, incumbent freshman Ami Bera (D) was trailing Republican former congressman Doug Ose by a tiny margin on Wednesday. It was so tiny that we graphed it to make the point that every vote counts (sometimes). Over night, that changed. At the beginning of the week, over 30,000 ballots were still to be counted, with Ose holding a slight lead. After counting those ballots, Bera slipped ahead. He leads by 711, out of more than 170,000 ballots cast.
In the 16th district, the Democratic incumbent Jim Costa took the lead over Republican Johnny Tacherra by only 86 votes on Wednesday. Neither race has been called.
If Bera and Costa can hold on, the state's Republicans will 1) be annoyed and 2) extend their dry spell of taking out Democrats by another two years. Or, really, four, as it's pretty unlikely that a California Democrat will lose in a presidential year. Unless it's Ami Bera, who will have won two races in a row by fewer than 4,000 votes in total.
This post has been updated to include the still-undecided race in the 16th.