How well was the gravitational field of Philae's landing spot known before the landing? I am referring to the absolute value of the pull, the direction of pull, and the tidal forces.
Rosetta had earlier this year been doing a triangular pattern of flybys to see how much the comet's mass would deviate it from a straight-line path.
According to this article they had a mass estimate with 10% uncertainty, which tells the overall gravitation to the same uncertainty if 67P is considered as a point mass. They may have refined the mass estimate since then, without Wikipedia keeping up. ;)
With a reasonably good model of the 3-D geometry of the comet body, and assuming relatively homogenous makeup, they could certainly make an estimate of the gravitational force on Philae as it approached the landing site. Both the model and the homogeneity would be subject to some uncertainty as well, though, so -- and this is just me guessing here -- there might be more like 15%-20% uncertainty in the estimate.