(This post is a mirror of a Hacker News comment I wrote.)
The "invalid tracking number" thing drives me nuts. Both Fedex and UPS do it. How hard is it to stick a record in the database the moment a tracking number is generated so that I don't get a "This tracking number is invalid" message? And it's not like it's invalid for a few minutes; I've had tracking numbers remain "invalid" for almost a day. Absolutely insane.
A lot of the systems involved were designed in an era before ubiquitous connectivity, when electronic data transfer involved calling a mainframe over the phone with a modem or sometimes even mailing magnetic tape. In that world the answer to “how hard is it to stick a record in the database” is “surprisingly complicated,” so these systems tend to work on the principle of “very eventual consistency.” I used to work for a company that had been doing eCommerce since the early 2000s, so can give an example of the way this happened: