As indicated above, one of the ways to corrupt an otherwise perfect model is to overwrite cells containing formulae. The Help facility will explain how. Finally, save your model to disk using a mnemonic filename that clearly identifies your model. The third step is to make sure you document your spreadsheet. There are a couple of reasons why this step is important.
Human memories are notoriously short and faulty.
If time has elapsed since you designed your spreadsheet, you may not be able to demonstrate these things satisfactorily unless you have adequate documentation to remind you. From time to time, your spreadsheet models will probably need to be up-dated or otherwise amended to take into account changing conditions. Perhaps more importantly, however, documentation allows other people quickly to become familiar with the spreadsheet and to modify it without unnecessary drama. This means that the spreadsheet can be maintained even after the original designer has changed employment.