If you've been building backend services with Node.js and MongoDB long enough, you'll eventually run into an error that looks something like this: BSONVersionError: buffer must be an instance of Buffer or Uint8Array or a variation of it thrown somewhere deep inside Mongoose's internals, usually when you least expect it — right after a routine npm install , a Docker rebuild, or a deploy to a new environment. No code changes on your end, yet suddenly your queries are exploding. ...