I've been meaning to write about this for a while, and finally had some time to do it.
Some projects have airdropped mainnet tokens to early testnet users, but these rewards aren't something the project teams promise.
The same wallet private key works on both testnet and mainnet. If a phishing site tricks you into switching to mainnet and signing, you'll lose real assets.
ETH on the testnet is just test currency; it's not the same as the $ETH on mainnet. The current price is 2439.9.
Have you ever misunderstood this too?

#ETH