Is Deflect a missable trigger that opponents can forget to pay, or must it always be paid?

Ruling: Deflect is a cost, not a trigger. In tournament play, intentionally not paying a cost to gain an advantage is considered cheating and can result in disqualification. Neither player can "miss" Deflect.

Sequence:

• When targeting a unit with Deflect using a spell, the Deflect cost must be paid
• For triggered abilities that target a unit with Deflect, the cost is added when those abilities are placed on the chain
• Players may decline to pay costs only for triggered abilities (not spells), which causes the ability to be removed from the chain before it becomes a chain item

Nuances:

• Deflect works differently than MTG's Ward mechanic, which is a trigger
• For spells like Hextech Ray or Smoke Screen targeting a Deflect unit, the cost cannot be declined
• For triggered abilities (like those created by Falling Star), players can decline to pay the Deflect cost, causing the ability to not resolve