Synchronoise has very peculiar effects in double battles, being both able to affect pokemon of unmatching type, or no pokemon at all despite matching type
https://www.smogon.com/dex/sm/moves/synchronoise/
The target is immune if it does not share a type with the user. Hits both foes and ally in Doubles matches.
Description leads player to believe that it will affect only all pokemon on the field with matching types as the user
Ex: Vaporeon will be able to harm an enemy or ally Tentacruel but not an enemy or ally Landorus with it's Synchronoise attack
Case 1:
https://replay.pokemonshowdown.com/gen7metronomebattle-950247360
Metronome battle
Floette-Eternal used Synchronoise, affected both Gardevoir Mega and Slowbro Mega
(Replicated outside of Metronome battle using same type matchup:
https://replay.pokemonshowdown.com/gen7doublesou-950252963 )
Case 2:
https://replay.pokemonshowdown.com/gen7doublesou-950249589
Bug Replication attempt one
Theorized that Synchronoise hit a Dual type with a match and was then able to hit all that matched that Dual type
Result: no damage
NOTE: This test raises more issues since it seems that if the first use of Synchronoise cannot find type matches it prevents all other Synchronoise from working even when there are matches
Case 3:
https://replay.pokemonshowdown.com/gen7doublesou-950252242
Bug Replication attempt two
Attempted to recreate original team matchup
Result: no damage
NOTE: This test seems to show that it checks the ally pokemon for a type match first, if it doesn't match it prevents damage to enemies even if the enemy matches the Synchronoise user's type
Case 4:
https://replay.pokemonshowdown.com/gen7doublesou-950253713
Attempt tried with both pokemon on attacking side having matching type
Result: Damage to all pokemon not using move, even those not matching attacker's type
Case 5:
https://replay.pokemonshowdown.com/gen7doublesou-950252963
Attempt eliminated a pokemon from the attacking side
Result: Damage to all enemy pokemon, even those not matching attacker's type
NOTE: This battle also is proof that the bug is not due to the Metronome battle format, nor the use of Floette-eternal
Case 6:
https://replay.pokemonshowdown.com/gen7doublesou-950254348
Using teams from Case 4, replaced one enemy to make sure neither of the enemy pokemon matched type with each other
Result: Damage to to all pokemon not using move, even those not matching attacker's type
Case 7:
https://replay.pokemonshowdown.com/gen7doublesou-950255010
Using teams from Case 6, replaced ally pokemon to make sure that only TWO pokemon (enemy and ally) have one matching type
Result: Damage to all pokemon not using move, (wonder guard activated on shedinja, counts as attempted damage), even those not matching attacker's type
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Edit: Application of Bug in the battle system
https://replay.pokemonshowdown.com/gen7doublesou-950268117
https://replay.pokemonshowdown.com/gen7doublesou-950275604
https://replay.pokemonshowdown.com/gen7doublesubers-950581862
Correctly built; a team with Synchronoise can spam a move with a Psychic Terrain boost that hits all pokemon on the field with base 180 damage.
Under normal circumstances such a power level is reserved generally for only Z-moves
(exceptions being super powerful moves boosted in terrain/weather with the correct abilities)
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Edit 2: Notes added to Case 2 and 3