SNIA's Cloud Object Storage community organized and hosted the industry first open multi-vendor compatibility testing (Plugfest) at SDC’24, in Santa Clara, CA. Most of the participants focused on their S3 server and client implementations.  Community driven testing revealed significant areas to collaborate, including ambiguities among protocol options, access control mechanisms, missing or incorrect response headers, unsupported API calls and unexpected behavior (as well as sharing best practices and fixes). The success of this event led to community demand for more Cloud Object Storage Plugfests at subsequent SDC events in Denver (April 2025) and Silicon Valley (Sept 2025). This panel of Plugfest participants will discuss real-world issues hampering interoperability among various S3 implementations, from both server and client perspectives.