We recently added a feature that used a library whose API we misunderstood. Long story short, if user A sends a request to our web application, the library caches some result, and that result may show in a response to user B's request. Needless to say, this is a security bug, specifically, data from user A leaks to user B.
Although it is well-known that web application should be stateless, the long dependency graph of such application makes the likelihood of some downstream library (or its bad usage) accidentally leaking data between requests non-zero. I can imagine this bug is possible with a wide range of web frameworks and environments (e.g., Django, .NET, NodeJS, AWS Lambda), since they all reuse the application between request to avoid cold starts.
What is the proper term for data leaking server-side between HTTP requests, due to an honest developer mistake? Terms such as session hijacking and session fixation seem to refer exclusively to malicious attacks.
Are there tools and method to test for such mistakes or detect them in production?