I have a template designed for Magento 1.7 installed on a 1.9 version of Magento...
1. First issue is that I can't proceed through the checkout and the other is that a customer can register, but when they goto login, there login won't work. I think this issue maybe down to missing form keys.
2. Second issue is that items aren't staying in the basket and then they seem to re-appear, from googling, I beleive this to be something to do with sessions or a badly configure varnish installation.