A small number of MUDs use a highly customized or proprietary network protocol built on top of telnet. Notable examples are Gemstone IV and DragonRealms, both by http://www.simutronics.com/.
If you play one of these, you may find it helpful to enable Simple Telnet Mode in Settings -> Advanced.
When Simple Telnet Mode is enabled, MUDRammer will ignore and discard standard telnet options.