My current hypothesis is that the server did not get restarted
after replacing the repository. Because we decided not to make
the instance ID part of the cache key, we could easily have
picked up cached format 6 data for the format 7 repository.
The lookup would start with a hit at the L2 DAG cache, mapping
path+rev onto a noderev. >From there, everything is accessed
using physical offsets until some item is not cached. At that
point, we would be trying to use an Offset instead of the Item
index to address the f7 data. Error.