100% agree with above; I spent some time setting up a table and then after about an hour's work, realized the table's text was ever-so-slightly lighter than the rest of the document text. Could not figure out what I had done so I started doing command-Z, undoing things step by step until the text turned normal again. The last command said "undo set global opacity" (or similar) and that led me to this thread.
I've searched for that phrase and/or term in the built-in Help and the online manual and I get "no results found" so I was pretty lost for a while.
So I have to work from the command-undo spot before I accidentally hit whatever key I hit.
And to build on the above, something global like this at least needs an "are you sure?" dialog pop-up before enacting.