We are also seeing the [ProcessingInstruction] tag in some of our canned messages.  We found that pressing the backspace prior to sending the canned message caused it to disappear.  From looking at the snippets.xml, it appears that this might be the result of additional html/xml tags from cutting/pasting Word formatted HTML when creating the canned message.  I am currently researching if manual editing of the snippets.xml will resolve this, and will report back when I know more.