Removed the assumption about get requests. Added a check if the client is online or not. If not online, just queue the request because we know the request will fail.

gistfile1.coffee

# Reference Backbone ajax function
_ajax = Backbone.ajax

requestQueue = []
requestPending = false

sendRequest = (options, promise, trigger=true) ->
    options = _.clone options
    if trigger
        requestPending = true    

    # Reference existing handlers
    success = options.success
    error = options.error
    complete = options.complete

    # Add custom complete for handling retries for this particular
    # request. This ensures the queue won't be handled out of order
    params =
        complete: (xhr, status) =>
            if complete then complete.apply @, arguments
            if trigger then dequeueRequest()

        success: ->
            if success then success.apply @, arguments
            promise.resolveWith @, arguments

        error: ->
            if error then error.apply @, arguments
            promise.rejectWith @, arguments

    # Send request
    _ajax _.extend options, params

# Sends the next request in the queue if one exists
dequeueRequest = ->
    if (args = requestQueue.shift())
        [options, promise] = args
        sendRequest options, promise
    else
        requestPending = false

# Queues non-GET requests to be executed serially. GET requests
# are executed immediately since they are assumed to not have side
# effects.
queueRequest = (options) ->
    # Since requests are being queued, the `xhr` is not being created
    # immediately and thus no way of adding deferred callbacks. This
    # `promise` acts as a proxy for the request's `xhr` object.
    promise = $.Deferred()

    if requestPending || !navigator.onLine
        requestQueue.push [options, promise]
    else
        sendRequest options, promise

    return promise

Backbone.hasPendingRequest = ->
    requestPending
    
# Override Backbone.ajax to queue all requests to prevent lost updates.
Backbone.ajax = (options) ->
    queueRequest options