Let me be the first PEA Souper to follow the blogging trend of linking to FAILBlog, and direct you to this real-life instance of the toxin puzzle.
The Paper Towel Dispenser Puzzle
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AuthorBen Bradley
AuthorBen Bradley 
A forum for discussing Philosophy, Ethics, and Academia
One can only assume that the sensor is mounted at a convenient height for optimal use … about 9 feet up the wall, I should think.
I suddenly noticed that the sign has a scope ambiguity. Which one makes a toxin puzzle?
1. Please try to (touch “No Touch” dispenser without touching it)
2. Please (try to touch “No Touch” dispenser without touching it)
To clarify: the first asks you to attempt something that is impossible; the second asks you to combine trying to touch with not touching.
Yeah, I noticed that too. I was thinking (2) was like the toxin puzzle. (2) also seems like the more charitable interpretation of the signmaker’s intent (Simon Rippon’s comment only makes sense if it’s (2)). It seems like most of the commenters at failblog were reading it as (1), the uncharitable bastards. Both interpretations raise questions about the nature of trying and the connection between trying and doing, so either way, it’s a cool sign.
It is a cool sign. My favourite is the paradoxical (is it a liar sentence?) sign in here:
http://my.opera.com/saysame/albums/showpic.dml?album=361896&picture=5197141