We have witnessed a number of cases when live migrations into Atlas were failing because the users had documents that violate the index key size limit (1024 Bytes). The users have also confirmed that they run MongoDB with failIndexKeyTooLong (so they could insert such documents).
One part of the problem is that people are getting poor advice to set that parameter on sites like StackOverflow.
The other part of the problem is that the documentation section that talks about failIndexKeyTooLong isn't really discouraging people from setting it unless there is a good reason to do so. I think we could fix it by adding a stronger message there.