test.rb

require 'csv'

puts CSV::VERSION

File.open('test.csv', 'wb') do |f|
  100.times { f.puts('あ' * 1_024) }
  5.times { f.puts 'a' }
  f.write("Invalid Byte Sequence in UTF-8 \x90")
end

CSV.foreach('test.csv') { |_| }

__END__
ruby 2.6.1p33 (2019-01-30 revision 66950) [x86_64-darwin18]
3.0.5
...
.../csv-3.0.5/lib/csv/parser.rb:277:in `rescue in parse': Invalid byte sequence in UTF-8 in line 99. (CSV::MalformedCSVError)