One place this is really problematic is serializing structures to JSON. I worked on a JS front-end at a company whose back-end was all Lisp, and I cannot count the number times the app broke with "TypeError: Cannot read property 'map' of null", because every single list that would normally serialize to an array would serialize to `null` when it was empty unless the person writing the service went out of their way to do otherwise.

At some point I wrote an API-validation utility almost entirely to help manage this problem.