Comment 6 for bug 1240794

I believe this can be reproduced taking a previously 'side-by-side' install as a base point:

1) Boot the installer with this as your existing disk,
2) Select the 'Something else' partition option,
3) Delete the root partition for the 'second' install (which for me is the first extended partition on the disk) by hitting '-',
4) Recreate this partition exactly as offered, selecting '/' as its mount point by hitting '+',
5) Attempt to tick the 'format' tickable.

Ubiquity will stop responding and you should find the python oops in /var/log/installer/debug.