I had 5 of each Radiant Echoes memories on two characters (one on each wow account) and was trying to compile them in one place, but the character that I traded them to didn’t have the warband bank access. I attempted to trade the full stacks back to my character on my main account, but it gives the error that I have too many of the unique item and the trade failed.
I tried to trade 10 Memory of Northrend, 10 Memory of Eastern Kingdoms, and 10 Memory of Kalimdor to a character that has 0. As a workaround, I had to trade each stack of 10 one at a time to get the trade to go through. Trading 5 of each in the intial trade was fine. I’m not sure if the unique 20 count is for all memories combined instead of individual memories.
I had also placed 10 Memory of Northrend in the warband bank earlier in the day on another character, but they completely vanished when I was checking the bank on the character that currently had warband bank access. I did check the bank again on the character that placed them in the warband bank after multiple annoying relogs and was able to see them and pull them back out on the character that original deposited them.