This error happens when you use UNetbootin to create an Ubuntu disk and it incorrectly adds a ramdisk to the memtest86+ boot option.

Until UNetbootin fix their code then cursor down to the “Test memory” option and hit tab and then at the boot options remove the “initrd=/ubninit” so that the command line is now just…

/install/mt86plus

and then hit enter and Memtest86+ will now run as expected.

My Ubuntu 14.04 currently has UNetbootin 585-2ubuntu1 and this quirk will possibly be fixed in newer releases but sometimes all you have lying around is an emergency install USB/disk so always good to know how to get around  a problem rather than downloading new code.