Originally Posted by
Charles Darnay
It is not that there is something wrong with the verse, but you are right to notice that it is not the usual iambic pentameter that we expect from Shakespeare.
If you look at the various songs throughout his plays, you will find that they are mostly in iambic tetrameter (four feet instead of five.) This comes from a long tradition of faerie songs written in this meter. Incidentally, children's stories also tend to be in iambic tetrameter.
The witches spell is in this meter to compliment the supernatural nature of the weird sisters. But if you look closely, these lines are not quite iambic tetrameter - they are missing half a foot. All this is intentional to create the weird, something is off, feeling that the witches provoke.