I've posted a few times here about my 100% non-hypothetical car kit that was evolved over the years as a result of being in the fire and rescue services. One of the most critical things that I have in that kit is a change of clothes and shoes. These are the most bulky and annoying things to deal with, and I've found that they are often the most important. Now, admittedly, I found myself in oddball situations like wading into rivers to help launch a rescue boat or arriving on-scene not needing turnout gear and not having rain gear handy and getting soaked, but all in all, a change of clothes, socks and shoes is really important. I pack zip-off shorts/nylon pants, a long-sleeve T shirt, a polar fleece sweatshirt, socks, underwear and a pair of sneakers. I consider these as important as water in my vehicle.

Other stuff in my NHCK (Non-Hypothetical Car Kit) has always been proven to be needed in actual emergencies and I've actually USED everything in it it for myself and others.

I'd say the most unexpected thing I've added to the kit over the years - because it turned out useful in many situations - is waterproof paper. So many times I needed to jot a note in wet/humid conditions, and having Rite In the Rain pads are super-handy. Cell phone chargers that use AA batteries are a must as well, and so are a bunch of lithium AA and AAA cells.