When you spawn a mob on a fence, the mob glitches and falls through the fence. Happens with both wood and nether brick fences; does not happen with glass panes or iron bars.
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Confirmed on both fences and Cobblestone wall in 1.8-pre2. This is probably since the mob is being spawned right on the top surface of the fences, but fences have a 1.5 block tall collision box with mobs, causing the mobs to be spawned inside the fence.
Duplicate of MC-88096 due to more detailed information.
Does this work on Cobblestone Walls or Mossy Cobblestone Walls? It may be because of their 1.5-block property.