If you unsort the records, when the record with blank id field was made ?
( because a record created before the creation of the id field has certainly the id field empty )
This won't help you to find the trouble source, but in general it is helpful to set the validation option "Not empty" for a field that's used as serial key. Should this happen again, you will notice it right away.
Odd. You might find all records, leave in unsorted order and see where the unnumbered one is: Is it first? last? somewhere in the middle? If it is first, maybe Daniele has hit the nail on the head. If it is last, maybe something has just changed? Is the sequence of numbers complete, or is a number missing? If so, is the gap where this record fits in the created sequence? (ie. could it have had its number deleted somehow?) These are just a few thoughts on trying to chase down the issue.