I was trying to install Lubuntu 18.04, the LXDE variants of Ubuntu, on an old Dell Inspiron 1525 laptop. After booting from a thumbdrive and choosing to Install Lubuntu, there were hundreds of lines of this output:
/init: line 7: can't open /dev/sr0: No medium found
At the end of that, it ends by dropping to the Busybox initramfs shell prompt.
Thanks to this Ubuntu bug report, I tried one of these solutions: the moment the above line started printing, I yanked out the thumbdrive and plugged it back into a different USB port. This laptop has 4 USB ports, so I was able to do this easily. Once I did that the error prints stopped and the install proceeded.