Start by using Google to search for xls and xlsx file types to quickly narrow down your search. Make sure you tell Google not to omit results by clicking the link "repeat the search with the omitted results included", or include &filter=0 in the address bar of the search as a parameter.
These examples search for xls file times that were indexed between Jan 1, 2016 and Jan 1, 2020.
site:sa.gov.au filetype:xls daterange:2457388.5-2458849.5
site:costco.com filetype:xls daterange:2457388.5-2458849.5
Any publically accessible spreadsheet, hosted by the author, is eligible.
Once you've found the error and fixed it, submit your contest entry in the form below. Make sure to Like or Follow us on Facebook - which is where we'll announce and feature winning entries.