When selecting multiple measure cells in the calculation pane in the tabular model designer, selected cells don't display correctly. Click a measure cell and then hold Ctrl and select multiple, non-adjacent cells. Note that the borders one each of the selected cells don't draw uniformly. With more than a few cells selected, some of the previously-selected cells don't even have borders so you have no indication of the selection. If you delete the selection, all selected cells are affected even though you can't see what you have selected.
Both SSDT/VS 2010 and 2012 behave the same way.