the RowBox problem appeared after changing to Mathematica 9.0. Some times when merging cells (with Ctrl Shift M), some lines of code disappear and a small Box pops up. When clicking on the Box the following message shows: "A RowBox with an invalid argument was encountered. The first argument to RowBox must be a list".
The disappearing lines are the real problem. This happens on and off, I can't see a reason for it.
If somebody has already solved that nuisance please let me know. Thanks E.F.