Joomla cache is on.
Cart module is displayed and cached with whole page.
This way the cart with product is visible for any other customer visiting exactly the same page - until cache time expires. It's suprising and confusing.
What is the best solution to exclude cart from caching? And why not to add such option to VM core/plugin.
I never run sites with the full J cache on.. endless problems with (as you found ).. cart mod, checkout, custom fields etc
Have seen some with conservative caching on...