There's a lot to parse in the sizzle reel Star Wars brought to Comic-Con. Rei obviously knows Han Solo well enough to help pilot the Millennium Falcon. Princess Leia is definitely in some sort of bunker. The First Order is definitely interested either in Jakku or the people (Rebels?) who live there.
But one of the most tantalizing bits is the running joke of lost limbs that's been going in Star Wars for almost 40 years. Poor C-3P0; he just can't catch a break.
Image Credit: Lucasfilm
Why is it a running joke? So many characters in Star Wars have lost hands. Anakin Skywalker, Luke Skywalker, Mace Windu, Count Dooku. It's the go-to disarming (zing!) move in a lightsaber battle.
So much so, Marvel has been paying homage to it throughout Phase 2 of the MCU. Check it out:
Other characters in Marvel that have lost hands include Nebula, Coulson, and Ulysses Klaw. But while Star Wars characters tend to only lose right hands – aka the hand they hold their lightsaber in – Marvel has been more lax on that rule.
So why would Threepio be sans left arm? Could be any number of things. Maybe there was an attack. Or maybe he's getting old and resources have to diverted to the war effort instead of repairs. We'll find out this December.