Im using QGIS 2.6.1 Brighton on an IMac with OSX 10.8.5. QGIS 1.8 worked on the same machine, as did QGIS 2.0. AFter installing 2.6.0 and then 2.6.1 I opened a file I'd used before and several queries were done. The attribute table had the selected items, but they did not appear on the screen as they had in the earlier version. When I tried to add the saved file to the map, it did it once and never again. The error message was that the shp file was not a valid data source. The shapefile does not appear to be saved. What am I missing?
Upon further experimentation, it appears that upgrading from earlier saved files into Brighton was the cause of the problem. I have regenerated shapefiles, imported them, joined them and queried them. The non-matches disappear from the screen and I am able to save the resulting layer and have it appear on screen.