I found a sack within a sack within a sack within a sack at Harnqusts and sold them. Thanks for the 12 Tria 
That was me

I've actually been doing some research on this bug. I've found:
Each sack within a sack takes on the accumulative weight-value individually, meaning you could have over 100 lbs within 10 sacks, each within each other, and the actual weight might only be 20 lbs.
Example:
First sack has 9 lbs of stuff in it = 10 lbs
you put it in the second sack so it = 11 lbs
you put it in the third sack so it = 12 lbs
Fair enough. But now, if you go back and take the second sack out of the third sack, and the first sack out of the second sack, you have 3 sacks (2 empty) with a total of 10 + 11 + 12 = 33 lbs, even though it should only be 12 (9lbs stuff + 3 1lb sacks).
In other words, the weight value for each sack doesn't reset - until you log off
or drop the sack on the ground and pick it back up.
Unluckily for me, while trying this bug, I had about 20 sacks all in sequence, with a gold ore in each of them (as I was actually trying to figure out a different bug).
This is how I got all the ores out:
Drop the sack on the ground. Open the context menu and 'examine' it (which opens a container window). Take whatever items are in this sack, and then take the next sack out of this sack, either sticking it into your inventory (if you can hold it) or dropping it on the ground. Continue this process until there's no sacks left within a sack.
Simple, right?

Until this is fixed, putting sacks in sacks is not advisable. Unless you just like to hear me say the word 'sack'
