If you've been working with Typed Arrays for a while in nightlies or betas (you're running nightlies and betas, right?), you need to know that there was a recent interface change. The other day Vlad pinged me to let me know that bug 630117 was about to land, and that it might break our code. Today I had other people telling me that the latest nightly was broken, and this was the cause. So if you're using slice() on a typed array, read this bug.