last_line_example.rb

result = []
last_line = nil

File.open('my_awesome_file_of_doom_and_cookies.txt', 'r') do |file|
  file.each_line do |line|
    if line =~ /expected_line/ && (last_line && last_line =~ /expected_previous_line/)
      result.pop
    end
    result << line
    last_line = line
  end
end

File.write('my_awesome_file_of_doom2.txt', result.join)