Bug introduced in 9.0 and fixed in 9.0.1
This is a more of a problem report than a question. When merging the two cells Cell1:
foo (*foobar*) bar
the line immediately before the comment together with the comment line embark on a journey into another universe, leaving nothing behind than a vicious red box and the farewell greeting
"A RowBox with an invalid first argument was encountered. The first argument to RowBox must be a list."