What happens if I turn off my console while it is saving data?

All console games warn players at the beginning of any game:

Please do not turn off your console when you see the saving icon/indicator.

What if I turn it off or there occurs a power outage while saving data? What are the chances? Is it for sure that there will be a harm in every case?

You could either lose or corrupt the data you are attempting to save. If the game is overwriting an existing file, this could mean that you will not be able to recover it.

Additionally if the console does not have a method for deleting the corrupted data you could wind up with inaccessible portions of your storage device.

It will mess up the ph balance in your pool. (Ok, not the last one).

Very best case? Your game won't save.

You also run the risk of corrupting your game files, which means that you would have to start your entire game over from scratch.

People have also reported that you can corrupt the entire hard drive of your console, which would mean buying a new console. This is a rare case, but it isn't something I would personally want to risk.

The same thing that can always happen when power is lost in a write operation; the data you're rewriting (your save data) can be corrupted, which is generally permanent.

It won't always happen, and there's usually reasonable protections made to stop damage from being done, but it's still not a good idea.