There are cases where a collection dropped from the _mdb_catalog reappear. The hypothesis is that the following sequence performed by a secondary:

1. Create collection
2. Complete a background index on the collection
3. Drop the collection

can result in the collection reappearing in state 2 when those individual updates are pushed to WT's lookaside table. The common manifestation is that a restart of the mongod process will fail with fatal assertion code 34433 because the _mdb_catalog refers to a table that was dropped from WT.