Inconsistent numbers in your presentation are a blow to the credibility of your story, and distract the audience as they are trying to figure out why the sales on page 19 differ from those presented on page 5.
Calculation errors and typos
Forgotten updates to the latest version of the model for some slides
Different sources of market data
“Footnote details”: certain numbers include something, others not (sales tax, etc.)
Last minute manual edits by someone without access to the spreadsheets
Some of these are not even “your fault” and can be perfectly explained. How to prevent them?
Decide on one view of the world, picking one market data source to go with, maybe with some adjustments, but that’s it. During the analysis phase of the project, ambiguity is OK, when it comes to presenting your story, ambiguity creates confusion,
Put all data in one spreadsheet, and link the different components of your model: if you forecast more sales, market share must go up if the market forecast does not change. More sales, means more sales reps, etc.
For each slide in your presentation, create a small worksheet, or part of a worksheet, that pulls the numbers from your model, and rounds them correctly. Nothing else should be in this worksheet. A column chart for 10 years: just 10 numbers in a row, nothing else. No need to dive into a big worksheet, dig up the numbers, round them. All possible sources of errors and inconsistencies.
Let no one touch the master version of this model
The added benefit of the approach that it is now very easy to update a presentation with a new set of numbers. It might take a bit of time to set up, but it will pay off in the end.