As you reach for a pfennig lodged between two cushions on a couch, the act of reaching for it will spread the cushions apart, allowing the pfennig to drop farther.  Given infinitely deep couch cushions and an infinitely long arm, you will never be able to reach the pfennig.

And who wouldn't want infinitely deep couch cushions?

This principle is often confused in popular media with the HeisenbergUncertaintyPrinciple, which is actually about mutually exclusive pairs of state values for particles.

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This is also an example of "Zeno's Paradox."

''Sounds more like the punishment of Tantalus to me.''

Hm, no. I think it's just a joke.
* He didn't mean the joke was itself like the punishment of Tantalus, he meant that the joke describes something more similar to the punishment of Tantalus than to Zeno's Paradox -- which is probably true.