I had plans in G+ to go to the Exploratorium at 2:00 on Saturday. I finished brunch early, so I arrived at 1:30. Google Now interrupted me at 1:45 to remind me to go to the Exploratorium.
It was in a calendar (this case G+). It's not going to know you don't want a reminder for something because it's not a human and it can't read your mind.
I'd rather have it remind me of things that I don't want to be reminded of than not be reminded of things I do.