When ColdBox is serving an event from the cache, it outputs the content right away. (link) Once the output has started, you can't add headers to the response. This specifically affects any postProcess interception point handlers. The writeOutput should not be called until after this interception point is announced.
Idea from , perhaps also add a preProcessExempt and a postProcessExempt flag to the cache annotations.
Idea from , perhaps also add a ignoreHeaders list to the cache annotations.