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Get them to describe everyday tasks. Examples:
Cross the road? Stop at the kerb, look both ways, then cross. You just got killed. You looked, but your actions did not take account of what you saw.
Make a cup of coffee? Fill the kettle (the kettle's already full of air) with water (from the washing up - it'll make you ill) etc.
Do one with the whole class, then divide them into groups each with a different task and get them to "pair programme" it. 3 minutes on the class task, 3 minutes in groups then 4 minutes presenting solutions. This will give them some feel for what real programmers have to look out for.
Update: I see hominid came up with a very similar idea while I was typing mine.
Update 2: Please let us know what you tried & how it went.