EC2 vs EC2-classic. I got badly burned (mentally) because I had to learn about creating EC2 instances using the company AWS account created before 2014, but then needed to create instances on a client account created in 2014.

Also: classic vs application load balancers - especially the health checks. That one drove me nuts for weeks!

AWS never seems to "deprecate" anything, but they do enjoy adding services which are very similar to other services but guaranteed to trip people (ie: me) up when it comes to the buried-in-the-documentation details.