Something that can always get me a bit frustrated is the choice of defaults used in applications.
Dates: Aside from Belize, Canada, the Federated States of Micronesia, Palau, the Philippines, and the United States are the only countries using a date format where the month is the first entry, followed by day, and lastly year (mm/dd/yyyy). To put to numbers that's about 436 million people who use this versus 6.35 billion that don't (ratio of about 14:1). Of that 6.35 billion about 3.8 billion use a date format where day is first, followed by month, and lastly year (dd/mm/yyyy — ratio of about 9:1 to the month first users). About 1.81 billion use a form where the year is first, followed by month, and lastly day (yyyy/mm/dd, roughly equivalent to ISO 8601 — ratio of about 4:1 to the month first users). (Note: these 1.81 billion have a slight overlap with the 3.8 billion due to some countries having two date formatting forms in use or due to two or more distinct scripts with different date formatting styles.) So using a format where the month is first is only confusing for the majority of the world's population. If you need a default date, use the ISO 8601 format — not only is it less ambiguous, it also allows for much better chronological sorting.
Temperature: Aside from Belize and the United States (I so far managed to find), the worldwide standard for temperature is Celcius, not Fahrenheit. If you are using Fahrenheit you are putting 6.48 billion people at a disadvantage solely against something like 313 million people. That's a ratio of about 22:1, meaning you put 22 people at a disadvantage for every one person you are trying to please.
Disclaimer: do note that this of course only makes sense if you are appealing to an international audience. If you are just targeting a specific country you will of course default to what they use. On the other hand, properly fixing your code to be i18n-ready is the way to go anyway.