There are a few block lists out there that pretty much throw in everything but the kitchen sink for domains to block, and aren't really tailored to work with a DNS-based solution like Pi-Hole.
In my experience, one of the worst offenders is https://adblock.mahakala.is. This blocks a lot of block list sites, as well as many reputable commercial sites like walmart.com (not just the ad-serving subdomains), etc. I deleted it shortly after I started using it.