Keychain style flashlight or headlamp, LED type, not incandescent: The keychain style is tiny and lightweight, the headlamp style is a bit bulkier but easier to use. You can get these for a couple of bucks or less at the local big box hardware store. The LED ones are lighter, more durable and last considerably longer. Very useful if you need to make a repair after dark! I keep a keychain style one as a backup as I normally have a headlamp in my backpack.
A pair of disposable latex gloves: Not absolutely necessary, but good for keeping grease and grime off your hands during a repair. Also, nice to have in an emergency when it starts raining and the temperature drops 20 degrees and you don’t have a pair of regular gloves. Just keeping your hands dry and the wind from directly blowing on them makes a huge difference in comfort.
Maintenance is the best prevention. “Prevention is the best medicine, and if you’re taking care of your equipment it’s super key. Bikes don’t just fail. If they’re in really good working condition and you clean them at night, and you’re religious about it, bikes are kind of like cars — they’ll start talking to you if something is getting worn or getting creaky or needs to be replaced.”