Running a $lookup/$project aggregation causes an invariant failure in sbe_stage_builder.h on 6.0.0-alpha:

{"t":{"$date":"2022-04-14T12:07:04.326-06:00"},"s":"F",  "c":"ASSERT",   "id":23079,   "ctx":"conn1","msg":"Invariant failure","attr":{"expr":"it != _slots.end()","file":"src/mongo/db/query/sbe_stage_builder.h","line":261}}

This failure occurs on Linux, OSX, and Windows and is completely reproducible. This $lookup/$project query has been part of the .NET/C# driver test suite for many years and runs successfully on earlier server versions.