Jigsaw blades are available in two major styles: U-shank and T-shank. Unfortunately, I was unaware of this fact when I bought my first jigsaw at a garage sale for $5. I mistakenly bought a package of T-shank blades for it when it really took U-shank.
Not wanting to admit that I wasted $12 on the wrong blades for a $5 jigsaw, I decided to use them anyways because the package said, "Fits all jigsaws". The blades did not fit properly and would slip out during use. I still insisted they were supposed to work, so I tightened the blade clamp harder... too hard it seemed, since I'd cracked it right down the middle.