This one has happened a couple of times to me...
1080/25P project (DNX36 media) with interlaced source material (so 100% motion adapter interlaced to progressive).
Clip has 4.3 reformat effect applied.
Step into effect and promote motion adapter to a timewarp, adjust to 200%
Step back out
Mark second half of clip and lift
Crash
v6.5.4 I think... on z800
Workaround is to remove motion adapter and apply timewarp