One I read of is the attachment of the gut to the back surface of the abdominal cavity, in common with quadrupeds, for whom it makes sense. For us, according to the source, it is a liability, and leads to complications in senescence.
From :
“Discover” And Here’s Why You Have an Appendix:
When you’re sick, it re-boots your gut with good bacteria.
by Josie Glausiusz
published online January 15, 2008:
“......it acts as a reservoir of healthy, protective bacteria that can replenish the intestine after a bacteria-depleting diarrheal illness like cholera.....”
Makes sense for humans to have one too it seems.