"There are lots of design mistakes, e.g. the appendix, no known function but if it gets infected it can kill you."
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I wouldn't be too quick to write the appendix off as a mere vestigial organ.
Seems it may play an up till recently unrecognized role in gut bacteria proliferation and possibly an adjunct to the immune system.
For more than you wanted to or needed to know about the appendix lol
http://en.wikipedia.org/wiki/Vermiform_appendix
FWIW I also wouldn't be to quick to call anything about the body a "design failure" given that we can't even create a cell from scratch with all we supposedly know(except for knee ligaments etc lol. There is a huge amount of redundancy built into the body, sophisticated automatic survival systems ie. heat management, reflexes, immune system, consciousness etc.
