I've just installed QGIS 2.6 Brighton using the OSGeo4W installation. The GRASS toolbox plugin has not, however installed as it had been with previous versions. Before, I was able to access the GRASS Tools via the tab Vector > GRASS, but it is now absent. Is this a change in the software or due to an error in the installation? If so, how can I rectify this issue?


Question: Random Forest to estimate land use in past with Landsat
Answer: I would look at the support of you individual classes. If support for a given class is marginal in your fit model, the error may propagate in very undesirable ways. 

I would also consider fitting a series of binary models and predicting probabilities of each class separately. You could then perform a sensitivity test on the probabilities and evaluate if there is multiple pixel membership (eg., pine forest transitioning into mixed). 

You may be having issues with trying to validate a hard-boundary classification where fuzziness exists in the class margins. A probabilistic approach may allow you to evaluate the changes in the ecological gradients.