400 Bad Request The plain HTTP request was sent to HTTPS port

Not sure if this is specifically WordPress but I've just helped a client move a site across from InterSpire to WooCommerce and some of the search results on Google are pulling up this 'error'...

400 Bad Request
The plain HTTP request was sent to HTTPS port

I've done a bunch of stuff to force HTTPS to be used but some of these errors are still showing up. Does anyone have any ideas why? Not sure what extra info would be useful?