Customers receiving order confirmation with another customer's details

Question

We recently upgrade out Magento 1.9.0.1 site to the latest patches through to SUPEE-10975.

We have started seeing a number of problems like so;

• Some backend orders show customer email address and customer group from a previous order
• Some customers have then received an order confirmation email with another customer's billing/shipping address, name and email address

I wondered if anyone else has seen similar issues?