Does Mongo have disappearing/phantom writes I have heard that Mongo has something like phantom writes.  For example, if I have a web request with the following db commands in a single unit of work:

a) update comments in article
b) update article attributes
c) update users comment_field

Questions:


Is it possible that only a and c execute, and b fails or never gets to the database?
Is it possible that only a and b execute, and c fails or never gets to the database?


Where can I read some more in regards to this, or is it just a myth?
Just to answer my question, in case people come across it:

1) Yes, it is possible for A, and C to be committed and B to fail.  Atomicity only exists on documents in MongoDB.
2) Yes, this is possible as well.

See more for reading here:  

http://docs.mongodb.org/manual/tutorial/isolate-sequence-of-operations/