Source: ABF Newsletter
I would appreciate your comments on a couple of hands that I appear to have misplayed (again). There is probably a theme, that’s why I put them together.
After 1
: 1NT, we got to 3NT. West led a small heart. I played low from dummy and took the first trick with the jack. When I played diamonds, they didn’t take the ace until the second round and, in the end, they turned their attention to clubs and I couldn’t get back to hand so it got a bit grisly and I went down.
East opened 1, I overcalled 2
, and we got to 4 without further opposition bidding. West led 2. I played dummy’s 10 and trumped East’s king. After drawing trumps, I tried running 
J, but East let that win. When I led a low diamond, East took my King and played back a diamond to West’s Queen. Both club honours were offside, so I lost two diamonds and two clubs. I though that it was a bit unlucky in the end, but you will no doubt have other ideas.
Yes, there is a similarity here because in each case your opponents have offered you an apparent gift. The divergence exists in that in your first example you should not have accepted and in the second, the gift was real but you may have failed to take full advantage.
In example 1, you have nine tricks as long as you can access four diamond tricks, two heart tricks, two spades and a club. You have received a favourable lead but not for the reason that you think. It is the most natural play to let this come to the AJ9 in your hand, but you later found that you were short of a vital entry when you needed it most. If you take K at trick one, you preserve the ace for its primary role – an entry to your established diamonds. The Ace-Jack combination gave you a cheap trick but it cost you a lot more later.
The second example is more problematical. It depends on where you think the minor suit aces are, and whether East has any hearts. It is possible that East has opened on something like:
Kxxxxx, x, AQx, 
Qxx,
but he would be likely to have more, including both minor aces, certainly likely in view of West’s pass. If you told me that you considered discarding a club at trick one, at least you get marks for thinking. If you ruffed without seriously considering your options to reach dummy later, you may have made an error. After taking your club discard at trick one, what can East play if indeed he holds the two aces and no trumps? He will either give you a free trick or access to the two spades, or both. On the bidding, it looks as though West has a flattish hand well short of six points.
You can most likely make the contract after ruffing at trick one. Draw trumps and play diamonds as you did. If East holds up, as he must, run J. Once West has Q, it is evident that East has both honours in clubs and will be endplayed along the same lines as above. You might have worked out that West did you a favour by not covering with Q.