Vidyard Events allow users to make adjustments to the HTML Source code, in order to perform tasks such as adding iframes.
You will see a message in the Event window that looks something like
NoSuchKeyThe specified key does not exist.
You may also notice that the source code has changed and includes parts that were not originally entered.
It is likely that there were some errors made when entering the HTML code. E.g. missing tags or incorrect syntax.
You will need to reenter the code, ensuring that it is correct when saved. A website such as https://validator.w3.org/#validate_by_upload can help ensure that the HTML is valid.