(Mis)Conception: "We lead ace-from-ace-king throughout the defense."
The Truth: Only at Trick 1. In the middle of the deal, lead the king from ace-king.

On opening lead, we would "never" bang down an unsupported ace--it is bad bridge. After, for example, 1-2-4, you wouldn't lead the A from A52--that is losing bridge. So, we define an opening lead of an ace as showing ace-king.  This method has been around for 50 years now and has become standard.

But--in the middle of the deal (any lead to a trick after trick one), we no longer should use ace-from-ace-king. Many players mistakenly think it applies after trick one.

Once dummy is visible, it is often a viable defensive strategy to cash an ace (without the king). While you wouldn't lead an ace from Axx, you certainly might want to cash it at trick 3. Maybe declarer took a losing trump finesse and you won your K. You looked at the dummy and saw running diamonds. You needed to take tricks. You quite often would need to bang down an ace (without the king). You don't want partner to think that you have the king when you bang down an ace in the middle of the deal.

Why does this matter? Suppose it is your play at trick 5 (against 2) in this position:

Vul: Dlr:	Q107
653		AKJ2
	984

Suppose you mistakenly lay down the A (erroneously thinking ace-from ace-king applies). Now, you try to get partner in (with some other suit) so that he can play another heart. However, he won't play another heart. He will think that you have only Axx or the like and were cashing the ace (hoping he could encourage if he had the king). You are denying the AK if you lay down the ace. The correct card is the king.  Then, when partner gets in, he will know you started with ace-king.

This is just one layout/example of many, but the bottom line is: Ace-from-ace-king is only at trick one.