foo = do
  A 1
  B 2
  C 3
  return () 

produces very different code than

foo = do
  A 1
  B 2
  C 3

for no good reason. Currently we check to see if the last clause is a return or something else, and then bolt a join in if it isn't return. But this check is too conservative. If it isn't a return but doesn't reference any of the variables above, you can still desugar it with (<*>).

We found this during a twitch livestream in front of an audience of 127 people. =)