Recenly i had an issue with one of my client site when designing custom theme. Whenever customer try to place an order from checkout page it sends ajax request to 'saveOrder' action and return 302 status and nothing happened. So customers cannot place order any more. Finally i figure out the issue regarding the form_key variable passed in ajax request. To solve this issue open following file in your custom theme: app / design / frontend / default / THEME / template / checkout / onepage / review / info.phtml and around line number 60, replace the following code:
with this one :
Hope this fix will help you..