I could only see this happening if that control was the only bound control for the record or you just happen to have the same code for the before or after update events for the control itself.
Also, I agree that it makes little sense to not use autonumber (long integer) because it occupies the same size/space on disk as a normal long if that's what you're using. You could only save space if you used number size of byte or integer and that save would be minuscule when compared to the 2Gb allowed for the file size. Not only that, you now have to worry about entering only unique values, and the number of records if using byte is very limited compare to the other types.
The more we hear silence, the more we begin to think about our value in this universe.
Paraphrase of Professor Brian Cox.