You could also use caching headers to know whether the client is about to update an object older than the current on on the server in the case where the form is

You could also use caching headers to know whether the client is about to update an object older than the current on on the server in the case where the form is being used to "edit" an object. Hopefully the server would respond to the If-None-Match header with the object version and give you a 304: Not Modified response... yay, you're free to upload the data! Otherwise you might want to notify the user that their changes are out-dated when the server returns the newer version of the object.