Affects Version/s: Minecraft 1.9 Pre-Release 2, Minecraft 16w15b, Minecraft 16w43a, Minecraft 1.11.2, Minecraft 17w13b, Minecraft 1.12 Pre-Release 3, Minecraft 17w50a
Fix Version/s: None
Environment:'os.name' = 'Windows 10'
'os.version' = '10.0'
'os.arch' = 'x86'
'java.version' = '1.8.0_73'
'java.vendor' = 'Oracle Corporation'
'sun.arch.data.model' = '32'
If the space selected for the player to land after passing through an End Gateway contains Iron Bars (maybe other transparent blocks too, but I had iron bars because they're dragon-proof), the player falls through the floor, which can lead to falling into the void.
Easiest way to reproduce: defeat the dragon, which will cause a gateway to be created at -56,75,77 (quite possibly over nothing but void). Build a small platform around the gateway, say 7x7 at Y=74. Place iron bars around the perimeter of the platform. Ender pearl through the gateway, and then ender pearl back.
When you return you will fall through the platform that you built.
Further testing confirmed that the gateway will attempt to place you as far northwest as possible, up to 5 blocks in each direction away from the gateway, depending on where it can find a solid block to put you on. If there is a solid block at -61,74,72, that's where you'll end up. If that block has an iron bar on top of it, you'll fall through.