If you boot or reboot and use a partition bootloader from QNX Neutrino on a QNX4 partition, an error will occur.
Because a driver sees the disk differently from when it was initialized, the secondary boot loader won't be looking for the root block, or subsequent blocks, in the correct place.
You'll see a message similar to this:
Press ESC for .altboot......
followed by a letter or character:
When these characters appear, the boot process halts.