Source: Bridge for Dummies by Edwin Kantar
Blocking your own suit brings to mind a true story that took place in a swanky London bridge club some 60-odd years ago. I call this story my own personal Gone with the Wind, and I share it to show you how treacherous blocking a suit can be.
In what was a high-stakes game, North was the pro playing with one of the weaker players at the club. The idea when playing with this young man wasn’t to win — that was impossible —but to hold the losses to as little as possible.
The day was scorchingly hot, and all the windows in the club were wide open. The pro happened to be sitting with his back to one of the windows. The pro tried to arrange the bidding never to let his partner play a hand if he could help it. The way the young man played a hand was just too painful to watch.
Then came the ill-fated hand to end all hands, shown in the figure. Going against his better judgment the pro allowed his partner to play the hand. The final contract, 3NT, was for nine tricks (a staggering total for the young gentleman). 
The opening lead was the 
K. After the pro put down the dummy, he went to get a drink but couldn’t resist walking behind his partner to peek at what he had. When he saw what the young man had, the pro was ecstatic. His partner was actually going to take ten tricks: seven clubs (by playing the 
A from the short side, and then a low club to the dummy’s 10, followed by five more winning clubs) plus three other aces. The pro was going to win a bundle. Unheard off he returned to his seat to enjoy the hand and also to figure out how many pounds (the game was in England remember) he was about to win.
The young gentleman took the first trick with the A and immediately led the 2. Instead of playing the high card from the short side (the A), he played the low card from the short side. By this one stroke, the young man had blocked the club suit and seven club tricks suddenly became two club tricks!
Here’s what happened: After playing the 2 and winning the trick in the dummy with the 10, the young man led a low club from the dummy to the A in his hand. Unfortunately the dummy now held five — count ‘em, five —winning clubs.
Poor South had no more clubs in his hand and no way to get to the dummy in another suit (rendering all the remaining clubs in North’s hand worthless). The club suit in the dummy was dead, totally dead. The inexperienced young man had blocked the club suit and compressed seven club tricks into only two club tricks, which is something you would never do, right? Nod your heed, for goodness sake.
When the pro saw the mistake the young man had made, he took his remaining ten cards and tossed them out the window. *Why are you doing that?» the young man asked. «You won’t need them anymore,» the pro replied.