When you have both an invisible spider, and in my case, a mule with a chest and saddle, the mule will turn sort of see-through as you can see the inside of the texture of the other side of the mule.
(If the resource pack makes a difference, you can find it here: http://www.minecrafttexturepacks.com/xenocontendi/)
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cannot confirm, is this still an issue in 1.8.8/15w31b?

Does this still happen in 1.8.8/15w31c? If so, please update the Affected Versions field.