What are the most common DIY projects that you should not do yourself? What must be hired out?
Edit: Just to be clear, what we're looking for here are the things that a regular homeowner might think they can do, but should not.
Things that look easy but are actually hard. Last summer my landlord decided to put a new roof on the house. They removed part of the old roof, and then the the rain and wind started. It took them 3 weeks to finish. Much of their time went to securing tarps overnight or bailing water out of the attic. An expert crew would have done it in 2-3 days, and avoided all that hassle. (We're still finding nails in the grass!)
Things that have to be done right from the beginning and are hard to repair later. Like plumbing in a poured concrete slab floor.
Or things that could be dangerous in non-obvious ways. Like gas lines, where an overtightened fitting can fail, and the whole house burns down in a hurry.