Bold/Italic Font in Email Subject Line Gives Error or Does Not Render

[#1]

Issue

Issue Description
An email with inserted bold/italic font in email subject line is not approving, with an error of "Cannot Update Stale Object," or does approve but the special font text disappears upon approval. 
 

Solution

Issue Resolution
The subject line in an email is intended to be plain text. 

Some have found success inserting basic symbols, or even emoji with specific encoding, though the rendering of these characters is dependent on the receiving server.  Related: Using Emojis in Marketo Emails