QUOTE(Gabbe @ Jul 17 2005, 12:49 PM)
No such case can exist, because several of the Melachos are mutually exclusive. For Example, one cannot erase and write, or plant and reap, or tear and mend...in the same activity.
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When one erases the corner of the daled thereby creatinng a resh, one is hayyav for both writing and erasing*. When one prunes a branch that one wants for firewood, one is hayyav for both planting and reaping, and when one tears material into a specific shape that also creates a garment, one is hayyav for both tearing and sewing/makeh be-patish**.
* According to Rambam, one can only be hayyav for one korban if the action had only one purpose - even if such action-purpose could be fit under more than one category.
** See Yerushalmi, and rishonim