I just discovered today through Google Analytics that I have been getting many, many 404 errors on my site. This, obviously, highly concerned me, so I began to investigate.
It seemed to me that every time someone viewed a post, a 404 error would be produced because the post was trying to access the post url with "null" directly appended to the end of it, thus triggering a 404 in the background. I do not think it affected anyone's user experience. See screenshot for an example of what I was seeing in GA:
I began to try to narrow this down to the culprit. I viewed a post, and it struck my eye that the "Like" button was cycling and not becoming "clickable." I became suspicious. I turned off the like button and checked my GA history to see when the problem began. I found that my GA history showed that I began getting intense 404s on Feb 26th. I checked the Jetpack release notes, and sure enough, the Like feature was released on Feb 26th, which I immediately updates and implemented.
I go through all that to say, I think your "Like" feature is somehow trying to access "url"+null and returning a 404 error on every page it is enabled on. Please resolve this issue. Thanks!