Hypothermia, natch. From lightbackpacking.com:

At Caribou Lakes Wilderness last year, backpacking buddies Duke, Wild Bill and I walked a 10-mile day-hike loop in t-shirts. When we returned to base camp, the sky was clouding up and Bill decided to go for a quick swim in the lake next to our camp.

The combination of cold water and no sun led to uncontrollable shivers, an early sign of hypothermia.

He quickly dried off and dressed in warm clothes. Duke and I started a fire and gave him some hot tea to drink. Experts say getting warm liquids down quickly increases your inner core temperature and staves off hypothermia.

Downside to going so light you leave the stove at home: no way to warm yourself up in said fashion.

For more: www.hypothermia.org/