Woah. Are you sure it touched the heatsink though? Is there a scorching on the fin? Looks like a 100amps went through that bit..
When I got the car off the track, the heat sink was definitely touching the bullet and it was scorched white. I couldn't replicate the sparking on the bench though.
This was running a 5s pack with a Neu 1512 3d. Doing practice laps at an indoor 1/8 track.