Rafael Monteiro discusses common mistakes made when using Cassandra as an infrastructure for databases. Some key mistakes include sharing storage across nodes, putting the commit log and data on the same disks, and putting a load balancer between the client and cluster. Other design mistakes include using Cassandra as a relational database when the data is not denormalized, having too many materialized views, and filtering by columns that are not in the primary key. Proper data modeling and running nodetool repair can help avoid issues. The presentation encourages using the right tool for the job.