It is possible that when using the Cart Picket Fence (PF-CART) a photogate will detect the edges of the (clear) plastic picket fence and not just the black bars. We have done some testing and here is what we found:

– We see this problem sometimes, especially on a picket fence that is a little scratched up. The problem usually does not happen with new picket fences.
     Cart Picket Fence (PF-CART)

– The problem is pretty much eliminated if you make sure the picket fence moves through the photogate so that is is farthest from the arm that contains the internal photodiode.
     Vernier Photogate – the internal photodiode is located on the arm to which the photogate cable attaches.
          Photogate (VPG-BTD)
     Go Direct Photogate – the internal photodiodes are located on the arm to which the accessory rod or photogate bracket attaches.
          Go Direct® Photogate (GDX-VPG)

See also:
Why does my photogate record a really high velocity at the beginning of a trial with a picket fence?