This is a bit hard to describe concisely in the summary, but relatively easy to reproduce :)

1. Set up a VM with two disks
2. Boot F19 installer, go to Destination spoke, select both disks, go to custom partitioning
3. In custom partitioning, blow away any existing partitions
4. Hit the 'create them for me' link
5. Select one of the LVs and edit the properties of the VG: set the VG to reside only on one of the disks, not both
6. Hit 'Update Settings'

At this point, the sizes of the LVs will shrink appropriately. The layout is all as expected and consistent at this point; if you were to complete partitioning, anaconda would create a single VG on one of the disks with all LVs in that. If it's the same disk that the non-LVM partitions are on, the other disk would be empty. So, we have all the space on the second disk as 'free' space. Now, let's trigger the bug!

7. Try to create a new mount point. In my test I created a '/home' mount point, since the autopart stuff didn't (as I have less than 50GB of space).

This causes the bug: creation of the new mount point will fail, with no detailed error message. I think what's happening is it's trying to add it to the existing VG but failing as there's no space.

There's various possible behaviours here. Just giving a more detailed explanation of what's wrong would be valid. Auto-creating a second VG on the other disk and putting the new mount point in that would also be valid (and what I was actually intending to do, in testing this). Giving an explanation of the problem and asking whether you want to shrink one of the other LVs or create a new VG on the other disk would also work, I guess. But just throwing a red error bar with no details definitely seems wrong.

To work around the problem, I just reduced the size of the / LV so the VG was no longer completely full. That allowed me to add a new mount point, which was created in the space remaining in the VG. Then I could edit the properties of this new mount point and change it to be part of a new VG, assign the new VG to the second disk, and increase the size of both LVs to fill the available space.