Being caught out, unprepared, in bad weather.

http://www.eptrail.com/rocky-mountain-national-park/ci_24887982/hikers-rescued-second-search-flattop-mountain-area-rmnp

A good quote from the above article rings true. “Most trails are not marked for winter use, so navigation can be challenging“. Even if you are experienced, a blanket of snow can make staying on trail a challenge. Make sure you have a good topo of the area, a map, a GPS, and the skills to use them. The $1 trail maps you buy at a trailhead, do not count. Large scale maps do not show land features, topography, etc. and can cause you to second guess your whereabouts like the interviewee said last week.

Resources:

Mytopo. Custom printed maps of your area: http://www.mytopo.com/

USGS: http://store.usgs.gov/b2c_usgs/b2c/display/(xcm=r3standardpitrex_prd&layout=6_1_61_50_2&uiarea=2&ctype=areaDetails&carea=0000000015)/.do

Nat Geo maps: I carry one of these because I cover such a large area:  http://www.natgeomaps.com/trailsillustrated