What I put in each knitting project bag:
yarn (if the project requires more than one yarn, I will probably put in only the two or three I will need right away. If I only need a little bit and the original ball is quite big, I will wind off only the amount I need for this project)
knitting needles, with different tips and cords if they'll be required soon, plus the "key" if it's that kind of circular needle
stitch markers for counting stitches during cast on. I usually use lightbulb stitch markers for this because they are tiny and don't create gaps between the stitches.
stitch markers for start of round, increases, decreases, repeats, mistakes that need to be fixed on the next pass, right side, etc. as needed, in different colours and/or shapes so it's easy to tell which one is which
small DPN for picking up dropped stitches, counting rows, transferring stitches from one needle to another, etc. The one I use most often is pink, so I call it the Pink Stick.
crochet hook for fixing dropped stitches (though often I'm too lazy and instead do it with my knitting needles, even though it usually takes me several tries because I can't remember how)
ruler or tape measure if needed (I have a short ruler tattooed on my finger, so it isn't always)
yarn needle for inserting lifelines and weaving in ends
scrap yarn for lifelines if I think I'll need them (I always need them, I just seldom think I will)
small scissors for cutting the yarn
moisturizer to adjust the grippiness of my hands depending on how slippery my yarn is
The bag itself must have a zipper closure because I will drop it and/or knock it over several times. I like it to also have little pockets, but if it doesn't, I have little pouches and boxes that I use to hold the smaller things.