Hi there,
Just been reading this thread and although explanations of very pertinent nature have been mentionned, I would like to humbly contribute, taking the risk of looking like a fool since my theory on principles of flight is many years old now.
The reason there is a MMO limit, say 0.82, is that beyond it, you will find some local areas on the wing that will reach or exceed Mach 1.
So I can easily see this local Mach value being largely exceeded if with the same speed, someone extends slats or flaps, increasing the camber of the wing, and having the airflow moving over the wing with higher velocity. The shockwave would be sufficient to disrupt, damage, or shear a component that wasn't designed to withstand such stress.
Feel free to correct if my statement is lacking accuracy, or has the wrong wording as english is not my native language.
Flex