Although I came across this using Spotlight, I feel this is a general issue;
I was setting up a new Instrument Key, and my design Layers were at a rather small scale which makes tweaking font Sizes difficult. As I have fixtures on multiple layers, I checked "All Layers" and set the scale to 1/4".
Well, to my surprise this changed the scale on the Sheet Layers as well, even though there is no way to change this scale while looking at a Sheet Layer!
I had to set All Layers to 1:1, then go back and reset all Design Layers to the proper scale.
Doesn't it make sense the Sheet layers should be locked at 1:1, and that scale commands while working in Design layers would not effect the Sheet layers? At least there should be a second check box in the Scale dialog making it optional to affect the Sheet layers...