Dannyboy - Quite agree that appropriate action is taken... But it has, hence my comment (probably stems from the fact I don't really like the title of this thread - The aircraft wasn't overweight but had 1200kg unaccounted for).
meleagertoo - Just in my experience and I don't call this 'overloading', but incorrect performance. Most shorthaul airliners flying out of moderate to large runways are frequently performance limited by the climb, not the runway length. Thus if a performance error occurs, usually there's little perceived problem due to both engines (hopefully!) running thus no actual issue with the climb segment. Going to longhaul aircraft, the performance is quite often runway distance limited - stop margins of 0m are frequently seen in the performance software despite moderate weights and 4km runways. Thus if the performance is run slightly incorrectly, then the perceived problem of the end of the runway is very apparent.