Changing LOOKUP table prices changes previous sales too!

Take the spreadsheet you just helped me with. I could accidentally change something in ProductLookup and not even know it changed a lot of things in Sales.

Locking in, as described in your previous message, sounds like the thing to do. But, since I was coming at the other problem from the wrong direction, I thought I would ask you if there are standard procedures that experienced Excel programmers use to prevent inadvertent errors.

Thank you,
Cane