One of the great features of digital cameras is the ability to change the ISO at any time on a shoot. The majority of my images are shot at 100 ISO but there are occasions when I change this to a higher rating, especially if the image is only for web resolution.
Have I ever forgotten to change this back to the lower setting? Yes! So now I double check, especially before a shoot that the ISO is set to the speed that I want for the particular scene that I am shooting. (My default is always set back to 100 ISO)
The easiest time to forget is when you have finished a shoot on a higher than normal setting, put the camera away and next time you start photographing forget that its on a much higher setting than is needed for the shot.
As best practice its good to check the cameras ISO setting when you put the camera away and again when you take the camera out. Also if you have changed the ISO for a particular shot on a shoot don`t forget to change it back when the higher ISO isn`t needed and don`t be caught out with unwanted noisy images.