If you're using Safari, downloading data as a CSV from Beacon can sometimes cause a problem. Rather than downloading the CSV file, sometimes Safari will open up the CSV file inside Safari, which looks something like this:

Pretty scary, right? Luckily, there's a simple solution! Simply Right click the 'Download CSV file' button in Beacon, and choose 'Download Linked File'.
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Alternatively, you can save the scary looking page by clicking File > Save As.

When prompted, make sure you set the Format to Page Source.

After saving, the file will have been correctly saved to your PC / Mac.