Usually, yes. This isn't necessarily the case, but is usually true.
When isn't it the case? A script for an on input can end up echoing text to the active window and halt the sending of it to the channel. There would be no error message in this case and it would appear to work normally except no one else would see your text. This could be a malicious script, or a scripting error. It's unlikely, but is possible... which is why I suggested testing it with another instance of mIRC to verify if it's sending the text to the channel or not.
