Interesting, we ran into a very similar thing at my current place where we have a CSV importer storing results in mongo. Importantly it also stores the errors that occurred during the import.

In our case someone uploaded a _huge_ CSV with some misalignment in the columns so every row had an error.

The resulting the mongo document was larger the max document size (16MB?), so it couldn't even save to the database.

Mongo is painful to work with and I feel like I keep finding more reasons to hate it.