Any company that wants to do business in China has to abide by the draconian censorship laws imposed by the country's government. With certain system configurations, apps that displayed the word "Taiwan" or the Taiwanese flag emoji would immediately crash the phone.
Anyone with that language setting wouldn't see the emoji but would instead see a square with a cross through it. He had no trouble reproducing the remotely triggerable bug, which crashed any iOS application that processed remote messages, including iMessage, Facebook Messenger, and WhatsApp.
The iPhone's notorious closed nature made analyzing the bug challenging. Even receiving the Taiwanese flag emoji on a chat conversation caused messaging apps to crash.
When triggered, the bug would crash any app where the Taiwanese flag emoji or the word "Taiwan" had been typed in for iPhones where the region had not been properly set.
After losing mainland China to communist forces in 1949, the Republic of China relocated to Taiwan.
China is the most populated country and the largest smartphone market in the world, and Apple would put an extra effort to please the large target audience. China claimed Taiwan as its territory, but Taiwan insisted on its right to self-govern.
After two+ years of being unable to type "Taiwan" or being remotely DOS'd anytime her phone received an Taiwanese flag emoji, the fix (kudos to my friend Josh S. for the idea!), was simply to toggle the region from United States to China, then back to US.
Luckily, that code won't cause a problem for non-Chinese iPhones anymore. Wardle reported the bug to Apple, which has finally issued a patch. If someone with their location set to China would try typing "Taiwan", their iPhone would end up crashing, as well.