Wrong vehicle numbers are always going to happen, no matter what. The issue is resolving them.

When I worked for a (non-EHI) franchise of a major rental chain, it happened regularly--at least a few times a week, if not quite daily. The difference was that we had a few procedures and tools at our disposal to avoid/resolve issues:

• We used paper cleanup tags stapled to the back of a paper copy of the rental agreement and the shift manager was responsible for doing a full audit at the end of each shift to ensure that the vehicle number attached to the contract matched the vehicle number on the clean-up tag.
• The shift manager was also responsible for doing a complete inventory of all vehicles on the lot. Sometimes a car that was listed on site was missing, and sometimes a car was present on-site that wasn't. The manager would then be responsible for tracking the discrepancies down, which were not infrequently due to an agent miskeying the vehicle number on the rental agreement.
• Vehicles showing as on-rent could not be assigned to another contract (it would pop up an error), so issues with an erroneously-assigned vehicle had to be resolved in real-time before the correct car could go out on rent.
• Managers also had the ability to reopen closed contracts to fix discrepancies (as long as they hadn't yet been posted to the ledger by the accounting office, which typically happened the following business day)..

(FWIW, I personally hated our reliance on maintaining paper copies in an age where all of the corporate stores of the major brands had moved to 100% electronic records, but I admit it did make resolving issues like this much easier, because there was a paper trail of checks and balances to make sure the data in the computer was correct.)

Not sure if National(/Alamo/Enterprise airport locations) still use ODYSSEY, but it sounds like the software isn't programmed to make resolving issues with erroneous vehicle assignment easy. It's programmed to try to get people on their way quickly (e.g. by closing the contract the vehicle was previously on-rent to if it's still open--which works if everything was correct and it just didn't get closed out, but not so much if the wrong car was assigned), and I think I recall that they didn't have a way to reopen closed contracts if one was closed in error (I could be misremembering, though). If they did have these tools, Qwkynuf 's issue may have been caught well before all the issues at return, and even if not, it could have been resolved much easier by reopening the rental that was erroneously closed in Texas, reassigning the correct vehicle number to the contract, and then closing the correct contracts out at the correct locations.