How do I determine why a MongoDB document is failing validation? How do I determine why a MongoDB document insert is failing validation? All I get back is a writeError that says "Document failed validation", which isn't very helpful.

(This happens often, and I'd like to understand how to properly debug these, rather than ask for help with a specific example.)
As at MongoDB 3.2, there is no feedback on the reason document validation failed: the overall validation expression currently evaluates as either True ("OK") or False ("Document failed validation"). Validation behaviour can be adjusted with validationAction (error/warn) and validationLevel (strict/moderate/off) configuration options, but this does not provide any further context for validation failures.

If you want to have more detailed feedback, the recommended approach would be to add validation logic into your application rather than relying solely on server-side checks. Even with server-side validation, many checks are best done in application business logic to minimize round trips to the database server and provide more responsive feedback to the end user.

For example, user input for a web app (required fields, field formats, ...) should be validated in the browser before being submitted to your application or attempting to insert/update in the database.

However, it does make sense to validate at multiple levels to ensure data quality and some context to diagnose validation failures would be very useful. There is a relevant open feature request you can watch/up-vote in the MongoDB issue tracker: SERVER-20547: Expose the reason an operation fails document validation.

For more information you may also be interested in Document Validation - Part 1: Adding Just the Right Amount of Control Over Your Documents. This highlights some of the general pros &amp; cons of document validation as at MongoDB 3.2, and includes a reference table for the outcome based on validationAction and validationLevel configuration options.