>>I would submit that 10b5-1 terminations when a merger is pending must have occurred more often than you suggest.
Someone would have to do a proper study to see if this actually happened. Turns out (contrary to my previous belief) there is no specific requirement to announce the termination of a Plan, although it appears to be the generally prevailing practice. You could study the pattern of previous sales to likely detect a non-disclosed termination.
Given the litigation that surrounds every acquisition these days, I would think that the class action lawyers would pounce on any plan terminations happening just before a merger.
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