I don't have a take up reel. The headstrike in the picture was on a single 11x17 poster. I would think this machine should be able to print 1 poster without a takeup reel.
Well, if the head strike was in the middle, that seams to me you had some kind of distorted bubble in the media in that area, it was not fat so the head hit the raised area.
Ok, to get rid of any humps, bumps or bubbles in the middle of your media you need to use a media take-up system to put tension on the media, to eliminate the hump.
If you don't have a take-up system, just run out about 12" of media past the front edge of the printer and attach some weighted clips onto the edge of the media to put tension on the media, that should eliminate the problems you are having.
I had a roll of media that took a UPS hit in the center of the roll, so I had a bump (hump) in the center of the media, before I had my take-up system ($300 on ebay), I would just attached weighted clips to the front edge of the media and the tension worked to pull out the humps...
Hope this helps