I've also run into issues where I wrote some code that worked with csv input, and told users they could just export their data from excel. turns out excel doesn't export in utf-8 by default, we had some weird issues until we figured that out.
Another issue is passing those CSV files across international borders - a CSV file that works in the UK (commas as separators and a decimal point) may not be readable in Germany (semicolons as separators and decimal commas) without some configuration.
