The Java doc for Cache#cancelUpdate says:

Note however that there is no harm in cancelling an update on a key that either does not exist or is not currently being updated.

Yet, the called EntryUpdateState#cancelUpdate throws an Exception if the key is not currently being updated:

if (state != UPDATE_IN_PROGRESS) {
            throw new IllegalStateException("Cannot cancel cache update - current state (" + state + ") is not UPDATE_IN_PROGRESS");
}

http://forums.opensymphony.com/thread.jspa?threadID=45046&messageID=91016#91016