Hmm, do you have a specific example? (Of the MUD and client?)
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.
[0] https://itunes.apple.com/us/app/mudrammer-a-modern-mud-clien...