Send text warning
Like the other person said - that line of text contained at least one character that cannot be transported to the Server with the currently in use "Server Encoding".
Mudlet can negotiate with the server over which encoding to use but many Game Servers do not support the telnet option (42 "CHARSET") to do that so you may just have to find out what the Server uses and avoid anything outside that range slipping into your output. I did code in a tool to analyse (a line of) the on-screen text - but it has to be enabled - and after selecting the bit concerned if you hover over the contect menu option it will report what is there::...
Answer to "What's a MUD prompt"
Importantly it doesn't end with a
<CR><LF> pair because, like in a terminal window, it usually contains some information appropriate at the time it is displayed and you will be typing in your input on the same line.
Well with the spell checking enabled I
Well, with the spell-checking enabled I was pointing out that, so the user could tell that a word was one that was in the "user" dictionary rather than the "main" system or Mudlet provided one it gets an underline but so that it is as different as I could make it from a word that is being flagged as a misspelling I made such words have a cyan underline which is dashed (compared to the wavy red one):
