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