A fun-to-troubleshoot "stupid Excel trick" that I ran into, arguably "caused" by the increased row limit in >2007 versions:
User typically drags formatting and "default" values down to row 65535 in Excel 2003-based "data collection" spreadsheet. Does the same thing in a >2007 spreadsheet, except they drag it down to row 1,048,576. The file size doesn't take a dramatic "hit" (since XLSX files are really ZIP archives), but performance goes in the toilet.
Knowing enough to unzip XLSX and DOCX files any eyeball the XML directly can often identify fun corner cases. (There's probably a ton of fuzzing fruit to be picked in the Office products using "malformed" documents, too.)
User typically drags formatting and "default" values down to row 65535 in Excel 2003-based "data collection" spreadsheet. Does the same thing in a >2007 spreadsheet, except they drag it down to row 1,048,576. The file size doesn't take a dramatic "hit" (since XLSX files are really ZIP archives), but performance goes in the toilet.
Knowing enough to unzip XLSX and DOCX files any eyeball the XML directly can often identify fun corner cases. (There's probably a ton of fuzzing fruit to be picked in the Office products using "malformed" documents, too.)