Bit of a strange one and in my years of Magento development have never come across this before.
When you add to cart in Firefox, chrome everything works as it should.
When you add to cart in safari then visit the site on another browser e.g iPad, iPhone you can see what cart items were added on the different browser. Discount codes and account details seem to be switching between everyone sessions so sometimes you are logged in as another customer. obviously this is a pretty big issue but i'm running out of ideas.
The server has Varnish which is running however I have turned this off which had no effect. Its running on Amazon EC2 if that's any help to anyone.
Magento version 184.108.40.206
It doesn't happen on our development server so that points to server rather than code / modules.
Any ideas on how to debug this would be greatly appreciated.