20% of the time your spell will land on the ethreal plain. been there done that. It dont help that summoned critter is hanging out on the ethreal plain, unless of course your fighting phase spiders.
You can't target the material plane when you're in the ethereal plane. While you're blinking, there's a 20% chance you will be in the ethereal plane when your spell finishes being cast.
I re-read the spell description and I would agree with Serum. There;s a 20% chance that the summoned or conjured critter shows up on th ethereal plan instead of the material.