I got to watch one of the world's great declarers bring home a tricky slam.  Take Barnet Shenkin's seat and see how you would do in 6NT after a diamond lead:

This is a poorish slam that looks to need a 3-3 heart split.  If the hearts don't run, do you have any chances?

Yes, if East holds the hearts and the spade king, there is an easy squeeze.  Knock out the ace of clubs, cash the spade ace, and run diamonds.  You hope to bring about this ending:

                     ♠98          

                     ♥K5

                     ♦10

                     ♣--

                                        ♠K

                                        ♥J1086

                                        ♦--

                                        ♣--

                    ♠Q

                    ♥AQ94

                    ♦--

                    ♣--

The ten of diamonds will do East in. 

So, you lead a club to the king, and a club to the queen, but both hold.  That's annoying!  They won't let you rectify the count.  What now?

It is time to play some diamonds, so you do.  On the second diamond, East shows out, discarding the spade six (standard carding).  Meanwhile West plays the diamond nine.  Suit preference?  Perhaps, or perhaps a meaningless card.  East throws the club nine and the spade five next, as you throw a spade and a heart.  You have reached a crossroads:

You can lead the club jack now, to rectify the count, but the ace and ten are still out, and could be in the same hand.  Or you could lead the last diamond, squeezing your hand a bit.  Your choice?

Conceding the club now makes the hand easy if East was 4-4-1-4 with the spade king, but you can still play for that after leading the last diamond, so that looks like the best play.  This is not clear though - nothing about this hand is easy.

So you try the last diamond as East discards the club ten and you throw the club seven.  It is safe to exit the club now, since only the ace is out, and this will squeeze East if he was 4-4-1-4.  Unfortunately, you haven't tested hearts yet.  East might well be 5-3-1-4, and you will think he was squeezed when hearts were 3-3 all along.  Leading the last club squeezes your hand before you have tested hearts.  Do you go for that or do you test hearts?

Barnet tried hearts, and West showed out on the third round:

The hand is probably makeable now - all you have to do is read the ending.  There seem to be three possibilities:

(1) East was 4-4-1-4 without the spade king, and you can exit your heart, squeezing West in the black suits:

(2) East was 4-4-1-4 with the spade king, and you can exit the heart to endplay East:

or (3) East was 3-4-1-5 and has stiffed his spade king, which you can now drop:

Barnet got it right.  And you?

(1) is impossible.  The opponents had you set easily, and have misdefended badly.  Playing for that position is rather silly.  Both (2) and (3) are quite possible, but Shenkin ruled out (2) for a couple of reasons:

First off, ducking the club twice is an easier play for East, who could see the squeeze looming, than for West, who had to envision partner getting squeezed.  And secondly, with the 4-4-1-4 hand, East might have discarded another spade, trying to look like a player with, say, ♠Kxxxx ♥J10xx ♦x ♣9xx.  If East had held the 3-4-1-5 hand, he had no real options in the defense.  The full hand was, as expected, (3):

At the other table, West led the jack of spades against a similar auction, setting the hand easily, so Shenkin's excellent card-reading won 14 IMPs.  Should West have found the spade lead?

That is very hard to judge.  Give North ♠Q98 and South the ♠A73 or even the ♠K73, and a spade lead would cost a trick. Indeed, South could easily hold four spades on this auction.  A diamond lead is much safer, so the real question is this - does this hand call for a somewhat aggressive lead?  Is a spade lead more likely to give them their 12th trick or set up our second winner?  I don't know the answer to that question. 

Here, they were struggling for tricks, and needed a squeeze to land the contract, but a spade lead breaks up that squeeze, in the simplest way possible!  

There is one thing I do know - the diamond lead was certainly the best esthetic lead, and paved the way for a beautifully played hand.