1. A computer-implemented method, comprising: an items manager receiving a set of one or more requests from a client for multiple initial data representation items, the initial data representation items representing initial data items in a data source; and in response to the one or more requests, the items manager performing acts comprising: generating initial placeholders corresponding to the initial data representation items; returning the initial placeholders to the client in response to the one or more requests; requesting the initial data items from the data source; receiving the initial data items from the data source; providing the initial data representation items to the client; and informing the client that the initial placeholders are to be replaced with the initial data representation items; the items manager receiving from the client one or more additional requests for requested additional data representation items; the items manager returning to the client additional placeholders corresponding to the additional data representation items; the items manager determining, for each of the requested additional data representation items, that the data source does not include a data item corresponding to the requested additional data representation item; for a first set of one or more of the additional data representation items, the items manager determining that the items manager does not have a record of a corresponding data item having existed in the data source and informing the client that a corresponding placeholder is a mirage; and for a second set of one or more of the additional data representation items, the items manager determining that the items manager does have a record of a corresponding data item having existed in the data source and informing the client that a corresponding placeholder is not a mirage and is to be deleted.