Today... My VTR with a kayfun and... a paper towel.
I was doing some studying at the university outside in the hot sun and my VTR was sitting on the table beside me, I picked it up to take a vape and I noticed a little juice had actually leaked out the fill hole of the kayfun onto the table, I thought I was gonna have to rummage through my backpack for something or just use a regular piece of paper to try to wipe the little juice off my mod, but I reached in my pocket and I randomly had a paper towel in my pocket, best luck ever, don't know why or when I put it there, but it was totally awesome.
I don't care to carry backup anything though, I usually try to have a somewhat charged battery and not almost empty tank when I leave the house, but if something does go wrong, I can handle not vaping easily enough.