Depends on your host OS, but most likely what happens is this:
While you are working & not saving, the visitor naturally sees the last version. Interesting things start happening when somebody accesses the page WHILE your save is in progress. This is really only probable, if you're upping a 300 kB page with a mobem line.
Anyways, whenever you write to a file, it is locked (in most configurations). This means, that it cannot be accessed while open for reading and anybody trying will get a 404 or other error page. On the other hand, when trying to write to a file open for reading the same might happen. On modern systems though, the file write is queued and will happen, if the read ends within certain time.
But to answer your question: No, the file will not be displayed with half of it from an earlier version and half from a later version. Instead, they will either get older version from some cache (there are many) or an error message. This is naturally the way it should happen and these kinds of situations are always a bit dangerous - for individual cases, anything could happen 😉
