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What does it break? This new default makes it behave as most users of it would have expected it too when writing their code. IMHO the users who require un-safe writes will be the ones to read this kind of news and notice and could alter it, if needed. I don't think this is true vice versa.

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One case that could break by this change is if you relied on insert's old default of being a no-op if an object with the same _id already exists. This can be used to efficiently insert a default document without race conditions. Example at https://github.com/RedBeard0531/Mongurl/blob/master/mongurl....

That said, it makes sense for this to be opt-in behavior rather than the default.

If people are relying on the current speed, changing the default to safe would break them.

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