I recently went through a ColdFusion upgrade. We were going from version 8 to version 11. Since we had previously tested the application with ColdFusion 10 we fully expected some hiccups.
Most of the trouble encountered was easy to find and fix, however one item really gave me some trouble.
Our application has a custom API that relies on RESTful URLs. We use the IIS URL Rewrite module to map API RESTful URLs to a ColdFusion API controller page. In previous versions of ColdFusion we'd used the IIS URL Rewrite module without any issues.
When we upgraded to ColdFusion 11 our API no longer functioned and provided very little error detail.