What tool or supply have you purchased that turned out to be the biggest waste of money? Consider cost vs. how much you use it.
For me it would be a Fiskars paper trimmer. Even though I bought the least expensive one at Michaels, around the neighborhood of $10, I could never get it to cut square, no matter how precisely I lined up the paper. It could be it was just faulty, it could be the paper would move when I slid the cutter. My Michaels doesn't take stuff back if it's been opened, so it's just sitting in my closet. I suppose I could still use it for cutting crooked stuff!