I'm using libcurl 7.18.2-8 in a C program, and I get the message "The requested document is not new enough" even if the server sends a file with response 200 OK. The problem I have with this behaviour is that the program depends on the HTTP response code to know if there was a more recent file or not on the server, and expects to receive a file when getting 200 OK. But in this case, curl doesn't modify the http headers, and makes a 0-byte file.
This behaviour happens if the server doesn't have a correct time, and is in the past. If the client sends a request with an If-Modified-Since header with a future date for the server, then the server can send a 200 OK response. I'm joining a file to show one transfer like this.
In my opinion, curl shouldn't check the file time in addition to the check done by the server, except if there is a good reason. I have checked curl code, and it seems that if there is a 304 http response received, libcurl already behaves correctly, and there is an additional code to check the filetime returned by the server (line 498 of lib/transfer.c on revision 7.19.3)