While light temperature has been mentioned here, I think the implications of tungsten or even candle light haven't seen enough emphasis. An 80A filter, which corrects for 3200K tungsten light, takes away two stops of light, and the reason it does this is because the blue spectrum is so weak in this sort of lighting. If you shoot in such light without filter, you think you shoot at ISO1600 according to what your light meter tells you, but as far as the blue sensitive layer is concerned you really shoot at ISO6400 or more.

I did shoot some indoor scenes with Portra 400 @ISO3200 last year, the first roll just straight with no flash and no filter, for the second roll I used very weak flash which only about balanced the ambient light. The difference in result was quite remarkable, the first roll was basically useless, whereas the second roll turned out quite nicely.