10: Thunder Cloud placement; Sohr Khai/Ramuh (Eden).

9: 3 stacks of DoT before eating fruit; Aurum Vale final boss.

8: Cannons; Stone Vigil HM

7: Knock-back Mechanics; Ravana/Sophia/Leviathan (EX and Eden)

6: Fear Itself (look away mechanic); Hydra/Calofisteri/etc

5: Healing to full to remove debuff (doom); Wanderer's Palace HM/Orbonne

4: Math; Construct 8, Ridorana Lighthouse

3: Burning Chains (tether mechanic); The Vault/misc

2: Pads (4 people); Labyrinth of the Ancients/The Weeping City of Mhach (Ozma)

1: Levitation is a STACK; Final boss, Syrcus Tower (I see people die to this...literally EVERYTIME I get this raid)

Stacking in general tends to be confusing for sprouts because not everyone is familiar with that terminology. I remember one time someone kept yelling 'soak! SOAK!' in chat then got mad when no one knew what in the Hell they were talking about. But anyway. There are so many more but these are the ones I see the most.