A problem that I've seen beginners run into in Haskell or PureScript a couple of times now is how to generate a List of random numbers. It's a common requirement for little games (which make for great first projects) to generate these, and it definitely seems to be a stumbling block.
Randomness is considered a side effect in purely functional languages, which means that to generate them you usually need access to Eff/IO, which in turn means we need to deal with Monads. And while generating a single random number is usually pretty easy with do-notation, the typical intuition beginners have built when going from single values to a collection is to use map, but that fails.