These days I don't really work with rails codebases, so I don't have any recent examples in mind to call upon. There's just a bunch of stuff you need to know about how rails works before you can successfully read/understand rails code -- I just remember seeing all this stuff in code and I was just thinking "where the f did this come from", and then realizing that the version of rails we were on had this feature that tied together these two parts with some obscure method.

I haven't even looked at Rails 5 or worked in a recent Rails 4 codebase so I'm sure things are much better but the code I remember the feeling of being perplexed so often at how a worked in previously wasn't my own so I can't go back and remember what parts really made me angry.