Blockstream CEO Adam Back:
What If One Bitcoin Isn’t Like the Others? Fungibility Debated At Scaling Conference | CoinDesk
"Some of the exchanges and wallets are using tracing services, and [if there is a connection to illicit activity] up to four hops deep away from you, they will freeze your account."
This threatens fungibility. Possible solutions are anonymity services like JoinMarket, Tumblebit and MimbleWimble
What If One Bitcoin Isn’t Like the Others? Fungibility Debated At Scaling Conference | CoinDesk