What are the most common defence mistakes?

Watching bridge online, here are some of the most common defensive mistakes I see:
3rd Hand Low
Not leading your partner’s suit
Not returning your partner’s suit
Not covering an honour with an honour
Leading an unsupported ace, just cashing high cards
Leading into honours in dummy
Leading a singleton trump
Leading 4th highest from a sequence or near-sequence (KQJ62, QJ973)
Giving away a ruff and discard
Not leading a suit because you can see a high card in dummy

[@sbpierce9987]

Concise information, clearly delivered. Thanks!

[@user-cn6ln7tp6i]

thanks

[@SmileyEmoji42]

Surely, not leading your partner's suit is only a mistake in in a NT contract. IN a suit contract you might be finessing against your partner with no possibility of later gain.

[@nagpurgal]

Awesome tutorial Tony!

[@tomau3946]

If someone has bid your longest suit, you still lead if if it is likely to be better than the opponent's, e.g. KQJ85 (The opponents have at most A and T between them.) But not if your suit is KT875, because the bidder may well have AKxxx (or better), which is stronger than your suit.

[@dofiah]

Very helpful, thank-you.

[@johnworf]

Lead the King of clubs on this holding and when partner plays the 8 clubs (showing precisely a doubleton club) you then switch to a heart and take the contract another one off for a huge top most likely at pairs!

[@benchapple1583]

Aha! Board 3. The best defence is to lead the king of clubs which declarer duly ducks and then switch to a heart beating it 2. A match-points perspective, I know.

[@archiewoosung5062]

Not leading into an honour in dummy & leading into a high card in dummy seem to be related, so perhaps they should be discussed together (might be, but I've only seen the schedule at this point where they're not adjacent)

[@MonteAmnah]

I thought you were supposed to lead 4th highest in a sequence if you are defending no trump bid?