tl;dr: MongoDB 4.2.6's transactions aren't full ACID, or even snapshot isolated. We found read skew, cyclic information flow, and internal inconsistencies, including transactions which could read their own writes from the future. Ooooh, spooooky!
Also transactions are allowed to lose data & read uncommitted, possibly impossible states by default, because why would you *not* want that behavior from something called a transaction. This was already documented, but I found it surprising!
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