I don't have every detail, so bear with me:
I am using Zencart 1.3.7 - store has taken over 1000 orders in 6 weeks, with "minimal" issues.
I just got a call from a customer who was attempting to go to checkout. Whenever he'd try, rather than prompting him for new login info (he already had an established account), he was shown as logged in, with billing information for someone entirely different.

Now - maybe it's coincidental, but I doubt it. The customer name/address showing up on HIS computer/browser was the info for the very last person to register on the site. In a different state. Unknown to the customer who called me. We tried several things, and the problem wouldn't go away, until we blew away his existing account and he created a new one.

That's all the info I know. Please advise what other information I need to provide.