REDU format ruling
Crystal Pair

Nearest-era rulings (Tengu, Sept 2011) — no REDU-specific archive exists.
Card effect (current official text)When a "Crystal Beast" monster you control is destroyed by battle and sent to the Graveyard: Place 1 "Crystal Beast" monster from your Deck face-up in your Spell & Trap Card Zone as a Continuous Spell Card, then you take no Battle Damage for the rest of this turn.
Rulings
- If you cannot place a Crystal Beast monster into your Spell and Trap Card Zone, the effect to prevent Battle Damage is not applied.
- You cannot activate Crystal Pair if you do not have any open Spell or Trap Card Zone spaces.
Rulings data courtesy of Format Library — formatlibrary.com. Spotted an inaccuracy? Flag it in the REDU forum — sourced change requests get reviewed.