Same problem here. When I load the page (even if logged out or anyone else loads it) a giant black check mark shows briefly on the screen. It’s huge and extremely bad looking. This is with the Dawn theme by Shopify.
Edit: I was able to stop loading the page just in time and grab the HTML
##
Item added to your cart
Updated Edit: OK I was able to kill this by going to Themes >> Edit Code. Then at the top just use the search field to search for “checkmark” and it should bring up the “icon-checkmark.liquid” file. If it doesn’t, try going to Snippets >> icon-checkmark.liquid. Then open the file, and I didn’t delete it as I don’t know what other files might depend on it. I would suggest not deleting it. I simply turned off the code by wrapping it in HTML comment code, like this:
Notice the at the first and last lines. Those are the wrappers to turn “off” the code. It seems to have solved the problem. Save the code changes and you’re done. Good luck!