Back when I bought my Canon 600D specifically for film making, I had everything set to manual. Most of the settings had to be locked anyway; 25fps, 1/50th shutter, ISO 100 unless you're indoors, and so on. This bit me in the bum during a shoot for a friend's film when I realised my first half-dozen shots of the day were at the wrong white balance setting.
So white balance went back to Auto and this served me well for a while, until recently when I was shooting The Wheels. Once again when I was reviewing footage (back home at the computer this time, far too late to do anything about it) I found it to be all wrong.
The cause this time was the mixture of light sources. At the location I had mistakenly assumed that the outdoor light coming into the room from two sides would be sufficient, but we ended up using a couple of domestic incandescent lamps as well. I knew the footage would look a bit orange, but what I didn't realise was that the Auto WB setting is taken from the brightest part of the picture, rather than the centre of the frame (as auto exposure is, when taking stills).
When Auto WB gets it wrong
The pictures above show Dave at the table, mostly lit by the incandescent lamps, but the brightest part of the picture is the outdoor scene beyond the patio doors. The Auto WB has set itself to suit the bright outdoor scene, and as a result Dave and much of the interior is a red/orange hue. This can be corrected in post but I found the shot above was so pushed to red it was tricky to bring it back to resemble something approaching normal without ruining the rest of the frame, especially when trying to brighten the noisy shadows. (My discoveries about noise in shadows is a whole other post.)
When Auto WB gets it right
This picture shows a different composition, which does not have an outdoor scene in the background; on this occasion the Auto WB has set itself to suit Dave and the interior rather than the outdoors. As per my usual workflow, some colour balancing was made to match the shot with the others, but you can see it required far less work, and therefore the image wouldn't need to be 'pushed' so far and degraded so much to achieve this.
Learning a lesson
When you're on an important shoot (as opposed to grabbing some shots on a day out with the family), white balance should enter your consciousness as one of those things you just need to check, like adjusting ISO to get the histogram looking right. Unfortunately white balance isn't something that's so visible on the camera, but luckily allowances can be made in post if the results aren't terrible. Perhaps a reminder on the shot list / storyboard would do the trick.