Extra medical supplies is definitely a big one. I always pack extras of the common ones, i.e. painkillers for headaches, plasters ranging from small to large and antiseptic wipes. Never know when they can come in handy!

A couple years ago a friend and I were planning a quick overnight trip through the New Jersey pine barrens. We were both a bit out of shape, but the pinelands are nice and flat and we're otherwise experienced backpackers so we were well-prepared to handle that.

Unfortunately the weekend we planned to go, that part of the state was on fire, so we scrambled to find a nice flat place we could hike into and camp for a night. We settled on one of the backcountry sites on assateague national seashore, not really understanding what that hike entails. Some of the reviews mentioned that a lot of it was on sand, which is the understatement of the century, because as it turns out we signed ourselves up for a 10 mile trudge down nothing but beach. And we got there at high tide so we didn't even have the harder packed wet sand to walk on, just loose dry sand, and no shade the whole way down.

We packed plenty of water for a normal overnight hiking trip that we'd usually expect to have a decent amount left over at the end (no potable water at the site), but we were guzzling it really fast, so we had a bit less than half left by the time we dragged ourselves into camp and made dinner.

We weren't so low that we were in serious danger, and the beach had plenty of day trippers along it so we could have bummed more water from someone if we really needed to.

Luckily there was another guy camping near us who had paddled in with a couple spare gallons he offered us and we happily accepted.

For the hike out we had low tide, and if we weren't already sore and tired from the day before that probably would have been the easiest hike of my life.

So anyway, now I take even more water with me that I did before.