Not mainnet but production testnet used by hundreds of people on a blockchain I worked for. I noticed a certain type of transaction returned me funds, so I set up a script to see how far it could go, next stop was mainnet. Eventually my script was blocked by the testnet relayer, so I inquired if we had a blacklist set up.
The respond was, "No dude, but you just blacklisted everyone."
The respond was, "No dude, but you just blacklisted everyone."