The most common way that Unicode is unintentionally inserted into a message is by composing a message in another program and then copying and pasting the message into the SMS content (either through MXT or third-party software).
Programs such as Microsoft Word and Microsoft Outlook often replace common characters such as 'and' with Unicode equivalents. These Unicode equivalents appear on more of an angle and are curvier than the standard encoding versions.
If you send an SMS from MXT and there is a Unicode character in your message, you will be alerted by a warning box informing you that ‘This is a Unicode message’. Removing any Unicode characters from the message box will close the warning box.
Note: One useful tip to avoid Unicode is to paste your SMS content as 'plain text'.
For more information on Unicode characters see Understanding Unicode.