The reasons for denying Gisele Lagace (Canadian comic book artist who draws Archie) entry into the US were down to the incomplete commissioned sketches she had in the car. Working on them while in the country would be considered working in the USA.
https://www.bleedingcool.com/2017/04/20/archie-comics-artist-refused-entry-usa-canada-sketches/
Another Canadian who is about to need an i194 waiver.