Worse, it's the necessary tradeoff of one of MongoDB's self-proclaimed benefits (asynchronicity) - it's not an edge case; it's part of the core reasons you'd switch to MongoDB (or at least take into account when making that choice). If that surprises you, it's because you haven't switched for the right reasons or haven't researched enough to know that every benefit has a tradeoff (and to know what those tradeoffs are).

To give an analogy, it'd be as if someone read this post and decided to use a SQL database solely because they care about write-durability... and then complaining when they "suddenly" encounter an error when trying to include an extra field with an INSERT on-the-fly. ('You mean SQL has fixed schemas??')