I'm one of the main devs at Froala Editor and I'm doing this for the last 4 years now. I've had lots of things to deal with while working with the browser editable fields.
I think there are 2 kind of hard bugs:
- Annoying ones. These are those bugs which keep on coming and coming and coming no matter what you do. Working with at the Froala Editor, I find bugs related to Chinese, Korean or Japanese typing as being the most annoying. They are almost impossible to test by writing automated tests and when you think you've solved one, there is another just around the corner.
- Challenging ones. You got to love them. They push your limits and make you think out of the box. Can't remember one off the top of my head, but it has to be something related to Regular Expressions. An infinite loop or something.