"Index of" can be your friend and the same with "web mirroring". Unfortunately, and also to the point of the talk/series you have to go look at this crap. It's *usually* not important. stuff like the /icons/ in Apache.
But every now and then pure gold will show up. so you have to go look at it.
So some examples of browsable directories that were not /icons/ :-)
yeah yeah but real world?! so for story time, we were doing a PT, the site had SQL Injection so were able to pull down lots of data but the sensitive stuff was *encrypted* so we were kinda stuck. Poking around further we found a directory with indexing enabled. what was there? database backup and a site back up with the decryptMe PHP function along with the current encrypt key :-) All from a "low" vulnerability.