This is caused by an issue with html5shiv (previously reported in this highslide dev forum post). When the shiv is present, the highcharts tracker overlay elements (22px wide) are render fully opaque instead of the being transparent as intended.
Try viewing this in IE8: http://jsfiddle.net/dM3ZR/3/ (funky grey lines).
Then, with the inclusion of html5shiv removed: http://jsfiddle.net/dM3ZR/1/ (all is well again).
This problem is resolved if the version from the innerShiv-branch is used (as mentioned in the discussion on the issue). Demo: http://jsfiddle.net/dM3ZR/2/
While waiting for the fix to get merged into the trunk, one option is to switch to the older 2.2 version which has been reported to not have this problem.
Issue resolved: The issue with html5shiv has been resolved (c993fe), and as stated by Emmett, the fix has been deployed on SE. We can all expect to see less freaky lines from now on (if you turn a blind eye on freehand circles!). Kudos to the devs at html5shiv and here at SE!