Reset Sale Number Sequence
If your sale number sequence has jumped by a large amount or if the sale numbers keep "jumping around", that suggests that the sale number sequence being used has started to conflict with sale numbers used in the past.
I.e. if you used sale numbers 40,000 to 50,000 in 2015 and somehow you recently picked a next sale number of 41,100
then Shopkeeper would start looking for "holes (unused numbers) in that sequence. It might find 41,359 then 42,401, then 42,571, etc.
The solution is to specifically reset the sale number sequence to a number in a range that will be free and clear going forward.
To review sales in order to select a suitable re-starting point, run an itemized sales report. Choose Full Selection so that on the right you can chose Both for Archived Data so that you also see the sales older than 3 years.
Sort by Date and run the report and see what sale number you should be on - and check that it's not in a number range used in the past.
The report could be very long, but you should quickly scan the entire report as the sale numbers cannot be in strictly numerical order.
Determine what a suitable number is and specifically type use that number for the next sale (e.g. even if that means going to 100,000 or 200,000 for the sale number.) and that will reset the number sequence going forward.
(The box for Increment the Next Sale number is usually checked by default.)
If your sales screen does not stop to let you specify the sale number, you should (at least temporarily) enable the option to make it prompt you for the number.
That's done from this screen in Preferences:
