Recently we migrated our website to Sitecore.NET 6.6.0 (rev. 130529). We have a custom handler (.ashx) to pull data from database in JSON format. We experienced Inconsistency in the response while executing handler. It worked for first two times and later it just responded with last response and request was not even hitting Sitecore. IIS Output caching (User-mode or Kernel-mode output caching in IIS) itself sent the cached response.
Later we came to know that IIS output caching is not supported officially by Sitecore – Sitecore doesn’t support ASP.NET Output Caching.