C-3PO's Arm...
This sequence, from 3PO's recent comic book Special, tells us how/why he ended up with a red arm in TFA:
http://s23.postimg.org/grqq6pbiz/C_3_PO_001_016.jpg
http://s23.postimg.org/m4fkktzff/C_3_PO_001_017.jpg
http://s23.postimg.org/rq1zow04b/C_3_PO_001_018.jpg
http://s23.postimg.org/doq03bwkb/C_3_PO_001_019.jpg
http://s23.postimg.org/on19lil5n/C_3_PO_001_020.jpg
http://s23.postimg.org/6bam4dcij/C_3_PO_001_021.jpg
http://s23.postimg.org/3wnbxolnf/C_3_PO_001_022.jpg
http://s23.postimg.org/84disoxp7/C_3_PO_001_023.jpg
http://s23.postimg.org/8epmps1p7/C_3_PO_001_024.jpg
http://s23.postimg.org/3xx9r3nh7/C_3_PO_001_025.jpg
http://s23.postimg.org/nibstvo2j/C_3_PO_001_026.jpg
http://s23.postimg.org/6j2uemcuz/C_3_PO_001_027.jpg
http://s23.postimg.org/4ttr6jx5n/C_3_PO_001_028.jpg
http://s23.postimg.org/nkvogprq3/C_3_PO_001_029.jpg
Not only is it a very interesting backstory, but the depth of emotion was beautifully conveyed... And yes, it's canon. 👆