I'me sorry to say: I experienced this too years ago.
This seems to happen under some circumstances that change the next number to a certain value (mostly already used) number.
We have try to find how and when this happens: however no success yet.
I advice to add after validator "automatic number", to add validator "Unique Term".
If automatic number goes wrong, there is second check to prevent errors. You are warned then.
I've used automatic numbering for many years and it's a very robust mechanism. By the way, Bert's right: when a record is duplicated the existing automatic number is erased and that's the way it should be! In earlier builds users could erase or overwrite automatic numbers and it was not possible to make the field read-only. So, in the old days duplicate numbers were quite easy to make. However, this was changed a long time ago. Automatic fields could be made read-only and still receive the automatic number from the system. Again, that's the way it should be because automatic numbering is useless when users can make up their own numbers.
There is however still a way to enter duplicate numbers. When records are imported, either from a window or a file, and the new records contain numbers that are read into an automatic field then these numbers are accepted.
There are circumstances that this is acceptable or even desirable, but it may lead to errors. Perhaps Cardbox should warn a user when he tries to enter data into an automatic field this way and prompt him for the desired action:
a. read and accept the imported data
b. ignore the imported data and add automatic numbers according to the settings in Automatic Numbering