Suppose there are 3 seperate melds on the table.
Blue 6,7,8, Red 5,6,7, Orange 4,5,6.
I made my initial meld and I'm holding tiles in my hand that would allow me to form new melds of 4,4,4, 5,5,5, 6,6,6, and 7,7,7, but I have two loose tiles left over (from the table) that can't form a meld.
Now what happens if the existing melds are rearranged to the point of no return, and I can't place the remaining two loose tiles?