There are already several irritating, get detected and its game over type missions, which just haven't really been fun, have been designed with really poor pathing for the AI in which to move around. Its not difficult, its near impossible to not get spotted by one guard, his patrol root seems broken as he starts off in one direction then spins around almost instantly and goes back, and in such a way that any route towards him he can see you... okay its fairly easy to kill him, and one of the optional tasks is kill no guards, with that one I simply can't find a way to not kill him and most of the time, be detected also.
The part I think you're talking about, might not be, with the human shield, maybe a cutscene decided to skip or something else happened, because I freed a prisoner, then it went to a cutscene, and from being undetected... feeling mid mission, suddenly I was in a massive fight with no indication as to how that happened. Just felt wrong, forced, and like the game is skipping through all the story/build up that made the previous stories so strong.
In an earlier part of that mission if its that(if that is the mission I'm thinking of) there is wagon part where the AI three times in a row started a fight, then just stopped with the friendly and enemy AI just stood watching each other and the mission wouldn't/couldn't move on. THere have already been, in 2 hours play max(mostly freaking stupid mission restarts and the like) been 3 missions I've had to restart to get to actually move forwards, and one multiple times.
Its ruddy badly put together, for the most part we're talking about making the engine run on windows, the scripting/ai/general code should be a snap to port over, if it works on consoles, why shouldn't it work on the PC, the thing there being, was this game as buggy on consoles and they just never fixed that either?
For bugs, gameplay more than graphical, I've not seen a game so fundamentally flawed since... Skyrim(and that was only one mission that had a major issue), before that Fallout 3 was a buggy mess, but just about workable.