From personal experience, resting your hand on the knob under heavy acceleration and adreneline (trying to kill a smart-ass in a SHO Taurus in my case) can cause the transmission jump out of gear, second gear in particular but I've also had it happen in third when accelerating hard out of a sweeping right hand on-ramp. This is, I was told, because the engine/transmission/shifter is moving around both laterally and radially under load. Your hand will hold the knob in a fixed place and the transmission/shifter movement creates the same effect as if you pushed/pulled foward/backward slightly on the shift lever, hence it pops out of gear. That's what I was told, may be total BS but in fact, since I was told this I've kept my hand off and have not had it pop out of any gear again.
The other reason is posted on RE's site, you can read it for yourself: http://www.rogueengineering.com/Merc...gory_Code=SNNN
I Googled it and found this answer check it out.... Do you rest your hand on the shift knob? - Bimmerforums - The Ultimate BMW Forum
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