Can you be a little more detailed in your description?  So far I cannot reproduce this problem.  Working in C++ I started with this code:
static void testGeneralCode()
{
	int nFoo;
}
Then I added a comment after the variable, giving me:
static void testGeneralCode()
{
	int nFoo;	// comment here
}
then using the keyboard I selected the entire line (I tried both Shift + Up arrow and Shift + Down arrow) and then pressed / to comment out the line.  The line is always commented out correctly for me.