In a competitive setting, can your opponent choose not to pay the deflect cost of a unit they are targeting, and what happens if they don't pay it?

Ruling: No, deflect is an additional cost that must be paid when targeting a unit with deflect. Failing to pay deflect costs is a serious rules violation (warning level), and the opponent cannot simply choose not to pay it and proceed.

Nuances:

• Deflect is not a trigger but an additional cost
• If a player fails to pay the deflect cost, it's a more serious violation than missing a trigger
• You might receive a failure to maintain game state warning for not pointing it out if it comes up later
• The opponent cannot play a spell targeting something with deflect and refuse to pay the cost