I have a Sony-Ericsson Xperia Arc S with android 4.0, the dev mode was activated by mistake by a non tech person. ( so now every touch/drag draws a line, and on the top it shows x,y coordinates )
I can't find how to disable it
If you just want that line / coordinate overlay gone, disable Settings - Developer Options - Pointer location
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To Disable Developer mode goto settings-> developer{ } then there is a slider on top of Developer options which you can on/off.