Parent is most likely referring to the telnet GOAHEAD character and its related SUPPRESS GOAHEAD protocol: https://tools.ietf.org/html/rfc858
I am the author of a MUD client for iOS[0] that, like most other clients, will capture and silently discard the GOAHEAD character. GOAHEAD is most common on MUSHes but I've seen them in a few MUDs as well.
Will try to put together some benchmarks for the cooler sketches & summary stats. Doing mutual information across two millisecond streams could be pretty convincing!