A critical aspect of euchre is that it is a partnership game. On every play you need to think about what your partner may have in their hand and how it could affect the balance of the hand.
This is especially true when you are trying to stop a lone call. A classic example; if you have the first lead, with four junk cards along with an off-suit ace. What might happen if you lead that ace? Maybe it will stop the lone, or maybe not. Who knows? However, this is a partnership game, and you need to consider your partner's hand as well.
In this hand, the dealer is in the North position and the turn card is the queen of spades. Bidding is passed back to the dealer, who picks the queen of spades and calls alone. (In this picture, he still needs to discard the 9 of clubs.)
Let's say you are sitting in the West position (LHO). What card would you lead to start the hand?
Obviously, you wouldn't lead the ace of spades as that is trump. Did you choose the ace of diamonds? That's exactly what the player in the West seat did. Follow along as I walk through the rest of the hand and see what happens.
East leads the Ace of diamonds. West follows suit with the jack and the maker(N) takes the trick with 9 of trump. Next the maker(N) leads the right. That lead picks up East's only trump. West throws off the 9 of hearts. The maker(N) continues, leading the queen of spades. East throws off the 10 of diamonds and West plays the queen of clubs.
For the fourth lead, the maker(N) plays the 9 of spades, with East seat throwing off the king of diamonds. However, look what then happens to the player in the West seat.
He is left holding both the ace of clubs and the ace of hearts and must choose between them! He has a 50 - 50 chance of picking the correct one, and here he guesses wrong.
Now let's consider what would have happened if East had originally led the 10 of hearts instead of the ace of diamonds: West would have played their ace of hearts. Now, on the last trick, both East and West seats would still hold aces and the lone would be stopped.
If the ace of diamonds had been the stopper, it would have been the stopper at the end of the hand as well. However, notice how leading it on the first trick affected the rest of the hand. And how it made stopping the lone a 50 - 50 proposition.