This error message is displayed for these reasons:
The banner command fails because it contains 240 or more characters.
A delimiter in the banner string can cause a partial banner to be applied to the device. For example, the message is displayed if the banner motd c This is to check banner c command is issued in the custom template using the letter "c" as the delimiter. When previewed, the "#" is displayed. The banner displays incorrectly because there is a word in the banner (check) that begins with the same character used as the delimiter.
This issue is documented in the Cisco Bug ID CSCsa06173
Use less than 240 characters in the banner string, and do not use characters for delimiters if the characters are also used in the text of the banner.