1. taking too much gear: 6 full size bars of soap, lawn chair, machete, canned ham, all real examples.
2. wearing cotton. I used to do it in the Sierra, but learned in the Cascades of Washington that the weather would bite you someday.
3. brand new boots for a backpack
4. planning to hike too far
5. planning to hike too many miles for the wife to have fun, not smart.
6. forgetting stuff: sleeping bag, boots, flashlight, tent stakes, tent poles, food, raincoat. I've forgotten them all.
7. heading out when the weather report is bad, bad, bad. I've never gotten used to backpacking in steady rain.
8. getting lost. some preplanning by having maps, is a good idea
9. not bringing enough food. that is no fun to go to bed hungry, and marmots are harder to kill than you would think.
10. hiking with people with different agendas than you. Personality and control issues, sharing of duties, bad attitudes.