When you insert a Create HTML Content activity, it automatically creates a HTML file in the .data folder of the automation project. The file will be named HtmlContent0.html
If you insert a second Create HTML Content activity, a new file will be created named HtmlContent1.html, and so on.
The problem is, if you copy the first Create HTML Content activity and paste it into another workflow, the copy will still be pointing to the HtmlContent0.html file. The same happens if you copy a whole workflow and rename it, the Create HTML Content activity will be pointing to the HtmlContent file originally generated.
That causes confusion when developing and wrong HTML Content being sent in email bodies, for instance.
Suggestion: every time a new Create HTML Content activity is generated, no matter if by inserting it, copying it of even copying the whole workflow it is contained in, a new HTMLContent.html file should be generated for it, avoiding the confusion and mistakes that arise from this situation.