Imagine the following - you need to speak for 60 seconds straight without repeating yourself, pausing, or going off-topic.
Now imagine there are three smart people next to you who will interrupt you the second you screw up because they want to prove that they are better at this trick than you are, and they're keeping score.
Now imaging doing this for a live radio audience.
And finally, imagine that this has been going on for 40 years.
I discovered Just a Minute (a.k.a. JAM) this weekend. JAM is a (mostly) weekly BBC radio "programme" hosted by Nicholas Parsons featuring four witty, competitive panelists in the best British radio tradition. If you're the sort of person who values the skill of speaking extemporaneously, it'll definitely ring your bell.
Nearly all of the shows aired over the last 40 years are archived in MP3 files that you can access via a dedicated Yahoo group. There are also numerous fan sites and blogs dedicated to the show. The gnomes of Wikipedia have done a nice job of summarizing the show and its history.
It's very difficult to speak for any length of time "without repetition, hesitation or deviation". But it's very funny to listen to what happens when people try.
Visit the Yahoo group and listen to a random episode, or if you prefer, let me know and I'll see if I can make an episode available here. It's brilliant.