HertogJan: Be sure to read the comments. Especially on the quests with exclamation marks as you might miss some.
From memory, among others:
- the guy on the stake at 1 of the bridges for entering Novigrad
- witch hunters entering a home just outside Novigrads wall
It's also possible to fail on 1 of Sigi Reuven's quests, but I can't remember the specific quests or what might go wrong.
It's the one where a player can decide whether or not to do that special favour for Mr. Reuven, to gain his appreciation.
I remember because my intention was to do just that..
Instead, I got myself entangled with some other matters at the same time.
Quest failed.
Me thinks 'twas because I proceeded with the main quest line too hastily.
Do pay attention. ^^