One of my standard programming interview questions is, "what's the worst part about your favorite {language, framework, ORM, ...}?" I find that if you've used something to its fullest, you usually have a measured and reasonable response to that question[0]. If the candidate isn't as experienced as they claim to be, often the response will be along the lines of, "Well, nothing really."
[0] Unless that something is Rails templating because dang, that is some crazypants there.
Templating can't be the craziest thing in Rails. In fact, I can't remember the last time it gave me any trouble. Are you just talking about how ERB specifically works, or how templates are invoked/rendered? Or is there just some insane metaprogramming stuff in its implementation (there always is with Rails)?
Mostly it's about how the templating process itself works. I once had a strange bug involving how a variable was (not) bound in the view, and I believe I lost count between 20 or 30 steps from when the controller tried to render the view until someone or another called ERB.new.
[0] Unless that something is Rails templating because dang, that is some crazypants there.