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Most gases that are toxic to breath also do nasty things to the skin when they're in the atmosphere at dangerous concentrations - and one is walking around without sealed protective clothing; sulfur and nitrogen oxides cause chemical burns when they mix with the water in perspiration, or on one's eyes, large amounts of ozone cause similar problems by different mechanisms. So my question is what are the key gases to use and, probably more importantly, at what levels to create an atmosphere that is lethal if you get a breath or two but won't burn your skin off or poison you anyway when you walk out the door without your moon suit?

Priority is given to gas mixes that are made up of gases that occur in/are produced by a biosphere almost identical to Earth's, so Water Vapour, Oxygen, Ozone, Methane, Nitro-oxides, Organic Carbon compounds, Carbon-oxides, Sulfur compounds etc... The best answer is the atmosphere that is most similar to our existing composition while being deadly to breath but otherwise safe for one to wear their birthday suit, apart from a breathing mask supplying safe air.

Pure nitrogen is harmless -- except that it won't support life. A breath or two will do no harm, but you won't even notice you're suffocating, because the carbon dioxide will clear from your blood as if you were breathing air -- but you won't be gaining any oxygen. You'll fall over unconscious after three or four breaths, and you'll die in four minutes (give or take whatever level of exertion was going on before, and whether you knew what you were getting into and could hold your breath for a while). The same would be true of nitrogen/oxygen, if the oxygen mix is less than about half what it is at sea level on Earth -- except it'll take longer to fall over, and longer to die.

Now, you wanted "lethal, but not corrosive" more or less. Hydrogen sulfide is biological in origin, more toxic than prussic acid, and although it has a very noticeable odor, humans lose the ability to smell it well below lethal levels, alarms are needed for concentrations as low as 5ppm. It claims a victim (or string of victims) on a fishing boat every so often, when refrigeration in the hold fails; a crew member is sent below to fix the system, doesn't come back, another goes to check on him, doesn't come back -- I've read about as many as eight people dying in this kind of scenario before anyone caught on.

A concentration of 800 ppm is considered 50% lethal, and 1000 ppm (that's 1/10 of 1%) is instantly lethal -- one breath will kill. As little as 100 ppm (1/100 of 1%) is considered immediately dangerous -- in part because this is the level at which loss of sense of smell begins. These levels can be formed by decomposition in natural settings, as long as there is a depression to collect the heavier than air gas.

Now, hydrogen sulfide will, eventually, form sulfuric acid, by oxidation and combination with water (vapor or liquid), but this is a slow process compared to its lethality.

an atmosphere that is lethal if you get a breath or two but won't burn your skin off or poison you anyway when you walk out the door without your moon suit?

take our atmosphere and remove all the oxygen, replacing it with an inert gas like nitrogen.

Asphyxia will ensue after a couple of breathes.

Asphyxia or asphyxiation is a condition of deficient supply of oxygen to the body that arises from abnormal breathing. An example of asphyxia is choking. Asphyxia causes generalized hypoxia, which affects primarily the tissues and organs. There are many circumstances that can induce asphyxia, all of which are characterized by an inability of an individual to acquire sufficient oxygen through breathing for an extended period of time. Asphyxia can cause coma or death.

This is the precise reason why, when using nitrogen to purge certain machinery, working in enclosed spaces can be lethal if proper precautions are not taken.

Otherwise such an atmosphere is perfectly safe to walk in it naked.

If you wonder what is the minimum oxygen concentration needed to sustain human life, you can refer to this

Serious side effects can occur if the oxygen levels drop outside the safe zone. When oxygen concentrations drop from 19.5 to 16 percent, and you engage in physical activity, your cells fail to receive the oxygen needed to function correctly. Mental functions become impaired and respiration intermittent at oxygen concentrations that drop from 10 to 14 percent; at these levels with any amount of physical activity, the body becomes exhausted. Humans won't survive with levels at 6 percent or lower.

My suggestion:

• 60% Xenon


• 
  20% Oxygen


• 15% Nitrogen


• 5% Carbon dioxide

Why Xenon?

Xenon is a noble gas. It has very few common chemical reactions, and is frequently used in situations where air is too reactive to be safe. It's safe to touch and even safe to breathe.

It's also a very effective anesthetic. Present-day doctors in Europe use it because it is remarkably side-effect free, if quite expensive. A few breaths of Xenon and you'll be out cold.

Why Oxygen?

20% oxygen is around the level we have in our atmosphere. This way, fire still burns and iron still rusts in your world the same as on Earth.

Why Nitrogen?

You have enough carbon dioxide to grow plants, but they need N2 in the air to keep the nitrogen cycle running. 15% is too little for the nitrogen-fixing bacteria in the soil, but your crew could have genetically modified soil bacteria for farming.

Why Carbon Dioxide?

This is how you die. CO2 is very stable and inert, but at concentrations of around 5000 ppm (0.5%), it becomes quite toxic. At these levels, it's so far above the lethality threshold that if you breathe this, you will die.

How does it work?

An astronaut is working on a new construction for the planetary base when her breathing mask undergoes a malfunction, disabling the low-air alarms. The astronaut continues working as her mask slowly runs out of air, and loses track of time. When the tank is almost empty, the broken alarm fails to remind her to refill, and without realizing, she begins to breathe the atmospheric air.

After three or four breaths, she begins to feel unusually drowsy. As soon as she realizes what happened, she holds her breath, but the xenon has already taken effect. Two shaky steps towards base later and she collapses on the floor, unconscious. Her brain, fooled by the apparently welcoming air, resumes breathing, taking in 13 times more carbon dioxide than she can handle. Instead of carbon dioxide from her lungs breathing out into the air, CO2 diffuses from the atmosphere into her alveoli, flooding her bloodstream with carbonic acid. Her blood pH rises out of control, and she dies of carbon dioxide poisoning in under a minute.

Pros:

• No damage to skin or clothing


• Lethal after a few breaths


• Behaves effectively the same as our atmosphere in most everyday situations


• You can grow crops with minimal modifications.

Cons:

• You need to explain how the hell the atmosphere became 60% Xenon. A comet, maybe, or it orbits a star with lots of Xenon?

Sources:

https://climate.ncsu.edu/edu/Composition

http://www.aragonvalley.com/en/effects-of-co2-in-humans/

https://www.ncbi.nlm.nih.gov/pubmed/17552896

http://anesthesiology.pubs.asahq.org/article.aspx?articleid=1945725

Carbon monoxide is a nice easy one. Colourless, odourless, non-corrosive, lethal at concentrations above 500 (or thereabouts) parts per million. It will oxidise over time to carbon dioxide in an oxygen atmosphere, so you’ll need some biological source to keep replenishing it.