Playing online with a pickup partner, I was subjected to the same defensive error ... twice! See if you can spot what that is. Obviously, I didn't say anything at the table and it appears that partner did not notice his misplay on his own.
You are East and you overcall 1S over 1C and the opponents bid 4H. Your partner leads a low diamond:
At the table, my partner (East) cashed a spade. I discouraged and he led back a diamond. It was too late now, of course. I think it should be obvious that declarer has a singleton Queen of clubs that he's just unblocked. The clubs are threatening: it appears that declarer's spade losers may vanish on the clubs. But ... your partner led a low diamond after you overcalled a spade. What kind of partner does not lead partner's suit? A partner with a singleton, that's who. Keep your Ace of spades as a entry. Lead back a high diamond. Partner ruffs, leads a spade and you lead one more diamond for down 1.
A few hands later, this comes up. You are North. Your partner (me) opens 1 heart. West makes a take out double. You bid 1S over that. East bids 2C, you bid 2H, West raises to 3C and I double. This should be penalty and you pass.
At the table, partner cashed his Ace of hearts. I discouraged and he switched to a spade, but it was too late now. They got out for down 1. Again, I think this should be obvious. Partner can not have three or more spades; he would have supported your spades (since your overcall over the negative double showed 5). Partner can not have a doubleton because declarer helpfully played the two. So, partner must have a singleton spade. Keep your Ace of hearts as an entry for a second ruff. Lead a high spade.
When partner's lead indicates that he wants a ruff, retain an entry to your hand.