What was mommy thinking?

Mommy didn't get a picture but I am still trying to understand what mommy was thinking.
Here we were playing one of my favorite games in the living room. The game is ... mommy throws my rope across the room. She says " get it" and I go running across the room and get my rope and bring it to her. She then says " give it to me" and I let her have the rope. She then throws it again. I love to play this game and will do it for quite a while untill one of my sisters get my attention and I have to go explore what they are doing.
Well on this day. Mommy didn't throw my rope! She threw her slipper!!! Now What was she thinking? That slipper doesn't taste or feel like my rope. It doesn't even smell like my rope.
I did go and get her slipper and when I brought it back she was laughing! She laughed and laughed but I didn't get upset because it was a happy laugh. She said that my face was priceless. What ever that is.
I did get some treats because I brought her slipper back and didn't chew on it and I gave it back to her when she ask me too.
But the question is.... Why did she throw her slipper anyway?!!!
Hmmm maybe it has to do with the Service Dog stuff?
Sally