commercial "mainframe" and "server" database management systems are not really susceptible to --
"The records show that our system caused a fatal error in our database and did not record some of our user actions maybe including loss of your registration details."
There is "logging" and physical redundancy - with the redeundant servers being on diagonally opposite coasts.
Also, the logging system is kind of neat. Briefly various database management system functions operate behind the scenes, preserving the value of the database and its contents. These "behind the scenes" operations include recovery logging and data replication.
In recovery logging, which is essential to eBay's system, as well as banking systems, credit card systems, etc. the system maintains a log or journal to keep track of all operations that affect the values of database items. This log or journal records the history of database changes in a secure way and contains details of all updates to the database. This recovery log includes "before" and "after" images of the state of an updated object. The "before" and "after" records mean that if it becomes necessary to undo some particular update, the system can use the corresponding recovery log entry to restore the updated object or value to the previous value.
The log consists of two portions, an "online" or "active" portion and an "archive" or "offline" portion. The "online" portion is the portion used during normal system operation to record details of the updates as they are performed, and is normally held on a local disk. When the online portion becomes full, or more frequently, its contents are transferred to the offline portion, which, because it is always processed sequentially, can be held on disk or in a tertiary storage device.
What is very important is that if the system crashes, it is possible to recover to a transaction consistent database state by examining the log and using various recovery algorithms.
The other "back up" is "replication." Replication is a process of maintaining a duplicate database, including database entries, and files, in more than one database at more then one geographical location. Replication involves copying (capturing) designated changes from one database (a source) and applying these designated changes to another database (a target). In banking, credit card, and auction systems the source and targets are can in different physical servers that are on different machines in a distributed network or different networks.
This is one of the many ways that banks, credit card companies, and eBay protect their data -- and user information. So, a fatal error in a database to not capture user actions is almost impossible. And, given logging and redundancy, complete loss of registration details or transaction details in even less possible.