The $_internalChangeStreamTransform stage raises tasserts whenever it encounters an oplog entry of an unknown type, e.g.

• here
• here
• here

Whenever one of these tasserts hits, we abort the change stream processing, with a message Hit a MONGO_UNREACHABLE_TASSERT!, in ChangeStreamDefaultEventTransformation::applyTransformation, which is correct, but not very helpful to figure out what exact oplog entry caused the problem.
This problem was uncovered by AF-8117 on r8.0.15, which ran into the exact same issue but left us guessing what exact oplog entry caused the problem.

For all these tasserts, we should provide a better error message with more details about the event (if possible), and also log a warning into MongoDB log for posteriority purposes.