I thought I was pretty good at recognizing them. When I'm bored; I participate in a lot of airdrops so I have a spam email account for that purpose. Last week; I received an email that I had won some tokens in an airdrop.
The organizer send me an email with a public/private key to an ETH address with ~$5,- in tokens on it (not ETH, different ERC20 tokens). There was no ETH to pay the fee to get it out. I transferred ~0.001 ETH from my own account to cover the transfer fees but a bot immediately transferred the full amount to the next address.
Looking more closely, I actually noticed several other small ETH deposits made that day and immediately getting transferred out.
It isn't the most profitable scam (most people transfer $1-10,-) but it was a new one for me.
NEVER transfer any currency to an address if someone generated the private key. It's tempting to see $5,- in an address when you hold the private key; but you'll just lose whatever you send there to cover transaction fees.
This is the first time I see a scam like this. But on the other hand this reminds me of the so called "Nigerian Prince" scam, which is similar to this one. You receive an e-mail from someone "belonging to" the Nigerian royal family. They say that they need to transfer a big sum of money and they want to use your bank account. They will give you 10% for that. If you agree, some time later they ask you to cover a fee, like $100, which compared to what you will gain in the end looks insignificant. After transfering that fee you won't hear from them again.