This code is actually not right.
If you don't use the eventCallback feature and allow the onSuccess to proceed (by returning true), you are losing traffic by definition — you just aren't noticing it because you haven't tested in enough browsers.
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If you have a moment would you mind explaining what the error actually does if it doesn't stop the cookie from getting stored on a browser? A Munchkin-logged activity requires at minimum 3 network requests: The Munchkin bootstrapper JS - a tiny script that manages Munchkin versions and sets up an event stack. The Munchkin core JS - the bulkier library that sets up the actual event listeners on the page, and sends requests for the tracking pixel. The tracking GIF - by default, the pixel is loaded w/Visit Web Page parameters as soon as script #2 above is loaded. #1 and #2 load from munchkin.marketo.net, #3 from a subdomain of mktoresp.com. The tracking cookie is created on the client side by #2 (it has to be, as it must exist before the activity is logged). The pixel is loaded via XHR (Ajax) in late-model browsers. Those browsers check after the pixel request has been sent and received -- known as a non-preflighted request -- to see if the remote origin (mktoresp.com in this case) has indicated that the response (the GIF) is permitted to be read by the requester. It uses the Access-Control-Allow-Origin header from the response to determine this permission. Your extension is tampering with the A-C-A-O header.
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