I noticed that Captain Falcon's Final Smash uses a sound effect from an F-Zero game that plays when starting a race. I think it's from the SNES game. PK Starstorm also uses the move's sound effect from EarthBound as well.
Pit and Dark Pit use the same sound effect as firing charged shots of the respective weapons in KIU when firing the Palutena Bow, Silver Bow and Dark Pit Staff. They also do the same sound effect as dodging from KIU when using any dodging maneuvering.
I like it when they recycle sound effects from the characters' source games. I think it adds authenticity to characters themselves.
Other neat details:
- When using Marth/Lucina's Final Smash, a life bar appears depleting to zero on the hit opponents. Said life bar is taken from Fire Emblem: Awakening.
- Upon activating Ganondorf's Final Smash, Ganon does the same animation as he does when he finishes the transformation in Twilight Princess (though notably faster).
- Chrom's one-liner ("On my mark!") when activating Robin's Final Smash is one of the possible phrases he says in FEA when a unit activates the Pair Up feature with Chrom as the partner.
- When using Link's side taunt with the fairy on the 3DS version, when the fairy gets behind Link, the glowing light will be visible. While this appears to be a bug, as it doesn't happen in the Wii U version nor with the Bottled Fairy item, it actually ends up recreating the fairies from Ocarina of Time.
- The trail left from Toon Link's Spin Attack is identical to the one left when using the Hurricane Spin in Wind Waker.