I just ran into another situation where one might loose code.
Enter the following three lines in an input cell:
a=1; (* comment *) a=2;
Then select the three lines and comment them out by pressing alt-/. That works fine. Then undo the action by pressing alt-/ again. The same bug as with cell merging turns up. Fortunately, here Rolf Mertig's very useful fix works as well.
The same happens when we use a blank line instead of (* comment *).