While hunting for bugs in http://opalrb.org/ I discovered this mind-boggling behavior in standard Ruby

locals.rb

def x
  2
end
p x # prints 2

x = 1
p x # prints 1
p x() # prints 2

# The above is... annoying, but makes sense. Here's the real WTF though:

if false
  y = 1
else
  def y
    2
  end
end

p y
# prints... nil !!!
p z
# undefined local variable or method `z' for main:Object (NameError)

# What happened?
# Even though 'y = 1' was never evaluated, *it still declared variable y*.

# Ruby's grammar conflates local variables and methods, and so the parser
# maintains a list of which identifiers are the names of local variables.
# When it comes across a bare identifier expression, if it's in that list,
# it becomes a variable reference (which cannot fail -- it returns nil
# if the variable hasn't been assigned to). Otherwise it is a
# method call, and if there is no such method then a NameError is thrown.

# Problem is, the parser obviously can't tell whether an assignment
# statement inside an if branch is going to be taken at parse time.
# So it *assumes that they all will be taken*.

# On the bright side, Opal managed to replicate this bizarre behavior,
# so good on them :D