I normally pack more than I could possibly need, but I do have that intense "better safe than sorry attitude." I try, within reason, to bring every piece of material or equipment that I can forsee possibly needing at any point. In most cases, of course, it would be ridiculous to, for example, pack an extra wig "just in case," but for me, it's a question of whether or not I want to drive 30 minutes back to Arvada to pick up a screwdriver or a pair of scisssors or pliers, especially after the con has already gotten underway. Then, not only am I missing at least an hour of the time that I worked so hard to look perfect for and to pay for, but at that point I'm also guaranteed to have to cruise the hotel parking lot like a vulture for another half-hour+ trying to wait for someone to leave, or pay $10 to park across the street for a few hours until something opens up. Especially if it's a repair that would take a grand total of 5 minutes or less. Infuriating! *RAWRtrampleDenverinarampage!* And THAT is why I always bring more than I think I will need!
(Plus, you might make the day of the person you meet who needs exactly that of which you brought extras!)
I always bring my pill-box full of naproxen, and my lovely lady's pain medications in case she forgets them. The way I figure it, a ruined con full of agony and dismay may be preventable!
ALSO: Band-aids, Band-aids, Band-aids. Blisters from new shoes, Abrasions from falling down stairs/in/out of elevator, something to cover the bite you sustained from a Great Dane at work the week before the con... It's a minefield out there, folks. Be prepared!!!
(Note from someone with no medical training: Puncture wounds (like dog bites) should be covered only with a dry bandage that allows air to circulate, constant attention to the wound and plentiful common sense. Never pack or cover a puncture with ointment or cream of any kind, as this can drive bacteria and dead or infected tissue deeper into the wound, which can obviously cause MAJOR issues!!!!)