Hah, ALL the time.
The girls at Joann's often ask me what I'm working on...most of the time my answer is, "Not sure yet! The fabric will tell me what it wants to be later, after it's met all its new siblings."
I like having a big enough stash that, when I get an idea, I have at least half of what I need on hand. Then I love shopping for the other half! LOL