The most frustrating thing in any software: Spell checking. I don't want it, I don't need it - why do folks force it and have no way to disable it? (Electron, looking at you)
The worst are the AppImages for Linux for apps where typing is a necessity and as I use multiple languages the spell checker is definitely not wanted. When there are no way to disable it, I often need to write my own software wrapper for those apps. FML.