I already have problems reading arrow/inline functions... for example where a method is called with a function that returns a function... if there is one more (async) call in there my brain starts smoking.
I start to feel uncomfortable when I see an explicit loop.
But that's actually very weird as I write Scala a lot where you use "for" (instead of "do-notation") for monadic computations.
My brain starts smoking when someone uses a "for" as an regular loop! That causes usually a few seconds of complete confusion. :-D