A single, well-constructed document can generate enough large keys to exceed the sorter's memory limit.

One solution would be to prevent this from happening and just fail an index build in that case. For documents inserted outside of an index build, we can most likely use the same global index build memory limit. We would need to be careful to ensure users can delete these oversized documents or update them to conform to the new limit.

This is not a new problem and has likely existed for as long as MongoDB has existed.