Option changing cart vs checkout - OpenCart 2.0.0.0

We're experiencing an issue where a product changes option "by itself" through the checkout process.

The products in question are shoes, and change from (say) US 11 to US 12 when you progress from cart to payment.

I have no idea why, and can't seem to resolve it. Any assistance would be greatly helpful.

OpenCart 2.0.0.0 was known to be buggy, you should update to at least 2.3.0.2 and probably the latest, currently 3.0.2.0.

Having said that, your issue may be because of sessions, so check your server settings around those.