The linked BF involved a fast count error that was detecting during shutdown validation. The basic sequence events involved having a background thread continuously performing the following sequence of operations on a collection:
Finding a document, updating it, deleting it, and then inserting it again.
This happened continuously while the node was restarted. In the original BF were we not able to pinpoint how the incorrect fast count came to be, so this test would be an attempt to recreate the conditions of the original BF in a simpler setting (unsharded colection, for example) and with more logging, so that if we do successfully recreate the incorrect fast count we will have a better vantage point for investigating.