Another big issue is inadvertent type coercion. I once worked on a system which consisted of about a half-dozen services and MongoDB. At some point we noticed

Another big issue is inadvertent type coercion. I once worked on a system which consisted of about a half-dozen services and MongoDB. At some point we noticed that a large percentage of the documents in a certain collection had somehow had a Date field coerced to String, which caused a bunch of stuff to not work correctly. That one took a while to track down.