If you parse code that produces an op_asgn node, such as e.g. a statement containing "+=" or "&=" etc., and the right hand side of that contains a string, and the string contains interpolation and a linefeed, the output ends up replacing the right hand side with a "<<- HEREDOC" but never outputs the HEREDOC.
Simplest test case I've found:
Test program:
require 'parser/current'
require 'unparser'
puts Unparser.unparse(Unparser.parse('y += "#{42}\n"'))
Output: