We've seen flatspun backside 900s and 1080s off the heels, but we're pretty sure that Sage Kotsenburg's backside 1260 off the heels at X Games is a solid NBD.
Spinning backside off the heels is hard. It feels un-natural and essentially involves throwing your shoulders in the opposite direction than you usually would when spinning off the heelside edge.
Sebastien Toutant landed a backside 900 off the heels over the summer, and Marcus Kleveland stomped a 1080 version of the same trick a short while back, but up until this point we've never seen a flat 1260 off the heels.
We've said it before, we'll say it again: Sage Kotsenburg is a wizard!