It's a bug that I have come across before. Another issue I had was Slic3r choosing a silly direction to do the bridging - bridging a slot that is 150mm long and 5mm wide, it would sometimes lay the first bridging layer in the long direction rather than the short direction. I'd then have to scrap the print and rotate it 90 degrees on the build plate to get it to bridge sensibly. As "digital dentist" has suggested, try changing the seam location.
Ensure however that the issue is not caused by your model having a feature (e.g. a hole) in the bridge. In that case the perimeter of the feature will be printed in thin air. If it has, modify your model to have a 1 layer thick membrane under the hole - which can then be drilled out.
Dave
Edited 1 time(s). Last edit at 12/03/2015 08:39AM by dmould.