I recall my limited time with Ruby and Rails being the really frustrating, seconded probably only by Tornado (either misleading error messages or no error messages whatsoever, requiring me to get dirty with the debugger for the most minor of things).

RubyMine had no idea about more than half of the Ruby code being written in the project. I couldn't grasp the concept of symbols in Ruby reliably enough. E.g. methods used either symbols or strings - I couldn't discern a pattern for which to be used when. This was the magic part of it - things seemed to work seamlessly but when they wouldn't, I had no idea where should I even begin to look.

Also every file had to be annotated for the strings to be treated as immutable. No ways to set it globally, project-wide.

It felt like a mess. On the plus side, there were gems.