Hi kimmo, yes like Brian writes above the Merge command can be used to fix up split edges like that:
http://moi3d.com/3.0/docs/moi_command_reference10.htm#merge

Select the 2 edges, push the Tab key and type Merge then push Enter.

It's not doing any harm though so it's not necessary to do this, and again like Brian says it will get split up again if you rejoin it to the vertical surface since the vertical surface's edges don't line up with its edges:



- Michael