It's possible when you don't realise that something will be cached (for whatever reason - incorrect verb, missing etag or cache-control headers, some cookie confusion), and suddenly everyone starts seeing one person's experience (hopefully only a slightly tweaked page, but at worst their account screen).

Turning the caching off is easy, but it can be really difficult to track down the cause after that because you tend to need a specifically-timed race condition, and even when you're sure you've fixed it everyone is really nervous to turn the cache back on again...