I'm not sure if anyone noticed this but Yusei performed a synchro summon of Stardust Dragon without a tuner. In episode 88, the materials used to synchro summon Stardust Dragon were Boost Warrior (level 1), Turret Warrior (level 5) and Card Breaker (level 2), none of which are tuners. However, during the synchro process, Boost Warrior acted as the tuner.