React: Each child in a list should have a unique "key" prop
I was getting this error seemingly out of the blue, and none of the usual search results made sense: I wasn't trying to make a list. Turns out, I'd improperly passed a doubly-nested child array to React.createElement(). A la: createElement( "div", divProps, aChild, moreChildren ); Where moreChildren was an array of nodes I'd made earlier.
WordPress Multisite 500 Issue
Was getting 500 errors whenever attempting to access a non-existent file located within wp-content/, wp-admin/ or wp-includes/ for a multisite — in my case, this was due to an upload that no longer existed (so a fairly easy thing to trigger). Hunting around on the web lead to no useful results at the time, so