Crochet provisional cast on
A provisional cast on is any method of casting on that can be undone later to release live stitches that can be worked from the cast on edge. This is my favourite method because it unzips so easily.
You'll need smooth scrap yarn close to the weight of the working yarn and a crochet hook in a similar sise to the needles.
Tip: The crochet cast on doesn't have to be provisional, done in the working yarn it's the perfect match for a regular bind off, which can be nice on something like a scarf.
Undoing the crochet cast on
At the end count how many stitches you have compared to how many you need. If you miss this half stitch or want to drop it off nothing will unravel.