downcase-region in marked rectangle does not downcase entire rectangle
and downcases things not marked:

On the following content in my buffer (*scratch*), upon marking the
inner rectangle consisting of "FG" and "JK" using C-return (cua-mode
enabled)

    ABCD
    EFGH
    IJKL
    MNOP

Applying `downcase-region` yields the following unexpected result:

    ABCD
    Efgh
    ijKL
    MNOP

I would expect "fg" and "jk" to be downcased, and not "fgh" and "ij".

Original report at http://emacs.stackexchange.com/q/9913/5154