What to do if White Balance is accidentally set at the wrong setting?

it is all about a habit for setting White Balance, before and after each shooting session; but if you forget, there are remedies:

I was shooting an indoor event of a Boy Scouts troop’s court of honor, under fluorescent lights from the high ceiling of an auditorium. I changed the white balance setting from “AWB (automatic white balance) ” to “day white”, to reduce the yellow patches on the faces, resulting from the camera’s registration of the ceiling’s light. This move was successful, and the “day white” setting also warmed up the skin color of faces, naturally (unless I shoot with telephoto lens from 8 – 10 feet away.) I learned this lesson from my previous shooting in the same location with AWB, under the same fluorescent ceiling light, that rendered the photos of people’s faces all having the yellow patches resulting from the traces of the down-casting ceiling light. On that occasion, I turned most of these previous photos into sepia or black-and-white in order not to show the yellow patches in colored photos.

At the end of each shooting session, after adjusting white balance for a particular lighting condition, it is also important to turn it back to AWB before storing away the camera, since the next shooting event may be under a totally different lighting condition. Suppose that a week later the same photographer was shooting portraits in a studio, and with the white balance still at “day white” from the previous shooting, the photos of faces would all have become way too red. How could that happen? Well, normally, without this oversight, the portraits would have come out just perfect, with beautiful and slightly defused natural or studio lights, if the white balance was matching this new lighting condition, with AWB or specifically selected white balance setting. With the oversight of shooting with the previous “day white” setting, the skin color of the portrait photos would be too red.

If that happens, it is not the end of the world. To correct the red color on each portrait photo, belated when the shooting session is over, one can use Photoshop to turn down the color saturation and adjust color temperature. Additionally, one can adjust contrast, and add more light to the shadows, while also adjusting brightness and highlight, if necessary. But it is much harder to remove the “shadow” on the side of the face resulting from the overly red color from using “day white” white balance setting, in color or in black and white.
Points to remember: 1) in indoor auditorium’s fluorescent lighting condition from high ceiling, adjust white balance from AWB to “day white”, or warmer, depending on each situation; 2) remember to turn it back to AWB next time, and 3) before shooting a new subsequent photo session, check if AWB is truly the ideal setting or not, by shooting a few practice shots, before session starts.