Not enough information to reproduce or its a bug that only occurs sporadically.
I've tried to reproduce this in Windows 7 and was unable. These are the steps I followed:
1) start Fusion
2) add a Loader to bring in some footage
3) add a PlanarTracker tool and view it in the left view
4) select the region to track and track it
5) connect in an additional Loader to the PlanarTracker's cornerpin input
5) scrub the timeline to try to provoke a crash
I was unsure which mode to put the PlanarTracker in, so I tried scrubbing the timeline in all 4 modes for about a minute in each mode (Track, Steady, CornerPin, Stabilize).