Last year people noticed that California Democrats had a higher percentage of the votes after election night than the final election night numbers. It wasn't a big deal until the SD-16 race this summer. Andy Vidak had nearly 52.0% on election night, but dropped under 50.0% when the final tally was in. Falling under 50.0% forced a run-off, which Vidak eventually won.
I've been asked what percentage does a Republican have to have on election night to win. Well, I'm happy to tell you. I tracked the competitive 2012 races by recording the margin election night and then the final vote. Of the 40 races I tracked, Democrats gained 3.3% or less in 33 of those races. So a Republican at 51.7% was safe in all of those. There were 4 races where Democrats gained 5-6%, with the highest the 5.8% Jim Costa gained. So a Republican with 53.0% on election night wouldn't have lost.
The GOP lost two races they led on election night, SD-5 and AD-36, both of which they led by 2.0%.