Dark corners of Unicode
I am not sure whether these are dark corners of Unicode or just complex language support. When dealing with text, you need to learn and educate yourself about text. International text is much more complex than localized text, but if you intend to support that, learn it. Sure, frameworks exist that hide a lot of the complexity, but then you hit a specific bug/corner case where the framework fails you, and since you are not familiar with the intricacies of the language-specific problem, you are clueless how to proceed and fix it, whereas if you invested the time to learn about it, it would have been much easier.A similar (albeit rather simpler and more limited) problem is calendrical calculus, where people who have little to no grasp of how to perform correct date operations do complex calendar applications and fail spectacularly in some edge cases.Call me crazy, but if you are dealing with text, have some time set for research before you start your development.