We often hear Cache invalidation is a hard problem in computer science and could cause bugs but sometimes, caching something that should not be cached is a potential source of bugs and security vulnerabilities. Rails has a built-in mechanism for Fragment Caching, which stores part of the rendered view as a fragment. For subsequent requests, the pre-saved fragment is used instead of rendering it again.
But this could cause some serious bugs! For example, we could have an S3 URL helper which generates a unique presigned URL for each product or one could write a form helper that outputs a request-specific auth token. In such cases, it is better to avoid fragment caching.
Way better than just adding a comment saying “don’t cache this.”