This problem concern the cluster mode and we use JBossCache.
--> Open an XWiki page in one browser one one instance
--> Open the same XWiki page in another browser one the other instance
--> Upload a document one the first instance
--> Refresh the second Wiki page
-------> The new uploaded document isn't here.
The problem is the same with an existing document (You get the old version of the document if you try to download it on the second instance).