That fretboard you think you know intimately can look a very different beast altogether, an evil twin, when the light is shining from a different direction!
That is so true, Rob! It has happened to me a few times...and one time I did not have enough light on stage to see my 7th fret dot.
My guitar has one dot (white) at the 7th, and I use a triangular piece of gold label foil at the 5th. I find the two shapes helpful, especially when the lighting plays tricks on me by glaring off the line of my bifocals and I start seeing 12 strings.
When the sun shines, bask.
Classical Guitar forever!