We have an odd checkout behavior that results in a cart confirmation error. Here's how it plays out. 1. Customer ABC logs in, fills cart 2. Session times out 3. Customer ABC goes through guest checkout instead of logging back in 4. Cart confirmation fails The resulting Sales Order has original customer ABC on it. There is also a new customer XYZ that was created for guest checkout. New customer XYZ's System Notes show as customer ABC then magically change to XYZ halfway through. Customer XYZ also has a "Contact: Individual Relationship" link to ABC. The cart confirmation fails to load due to a permissions error when customer tries to load ABC's sales order details.

With Cart confirmation, do you mean sales order confirmation after placing it? It seems a bug on the core, when the session times out, you are 'recognize' and probably it only changes after you actually logged in with another credentials but not when guest checkout, why? I'm not sure but it uses different service for logged in users and guest checkout, so it might be the reason, the guest checkout is not clearing or handling correctly the session. However, it is a really weird scenario in which a previously logged in user is going to then use guest checkout. Do your customers use this for a particular reason? Guest checkout is for first time and single time purchase.

Yea, the error pops right after the order submit event completes.

I see, I would take a look at the login service to see how they are clearing/setting the values for the current user and then check how the guest checkout works and create a custom module to add the missing logic to handle this scenario.