It's rare that I've had a problem with getting my reserved seats, however it does happen from time to time on some services and unfortunately often it's usually on those where the train is so full that the chance of seeing an inspector is next to none.
The biggest category I see, myself and other passengers having this issue with, are where people have reserved a ticket for an earlier train and missed it and believe their reservation carries them forward to the next train even though it doesn't.
Then less common, but still possible to see, is the peopel who say they bought an anytime ticket which costs more and I have an advance ticket so therefore the seat belongs to them and I will have to stand because I didn't pay as much as them. That happened to me a few weeks ago, but the guard just upgraded me to First Class.
In relation to Greater Anglia, not having reservations has been a double edged sword. If you get on at a terminus where the train is empty, then there's no issue whatsoever, but if you get on a busy service further up the line getting a seat can be almost impossible without reservations on some services.
The other problem with Greater Anglia MK3s at the moment unfortunately is because the coach formation is such a mix and match with some coaches carrying up to 4 different letters on them and basically the formation decided by Guinevere and a random set of balls, it's hard to work out which coach you are in, which one is the quiet coach, which one isn't, because the actual quiet coach at the front may not have a quiet coach label on it, with one in the middle which is not the quiet coach with such labels on, and the guard making an announcement that Coach B is the quiet coach which he thinks is actually the front coach, but is actually a middle coach, whereas Coach C is at the front, which is actually a quiet coach. Mix into this that the lack of reservations means that non quiet coach people are not being kept away from it like they did with reservations, and the whole thing is useless.