I checked out the metal plate under plastic SHAD rack and three of the four mounting holes have cracks from the hole, going out. I didnt see any inward cracks, but I don't want this tail bag to fly off down the road!
I have sent an email to SHAD and am waiting for their reply. I suggest that if you have one, you check the plate.

Is this the same metal plate that Murph sells for $50.00 to mount the Shad? 

Interesting.  I have the Shad 46.  I'll check mine just in case.

Shad won't make good. Their website has list of all the wonderful reasons why you should buy SHAD, one being guarantee, but I couldn't find any info on guarantee. Yeah, I know this thing is four years old, but this is clearly a defect on their part and a lot of companies offer lifetime workmanship warranties.
This could be a safety hazard as you can have a passenger resting on the bag while you are slabbing it and if the bag breaks off, they'll end up on the ground!
There is a link for warranty, but clicking it gets you a dead link.
Interestingly, i found a page that says that all SHAD stuff is sold as is, no warranty at all!
Here's a paste from it:
"
Warranty Disclaimer
This site and the materials and products on this site are provided "as is" and without warranties of any kind, whether express or implied. To the fullest extent permissible pursuant to applicable law, www.shadonline.com disclaims all warranties, express or implied, including, but not limited to, implied warranties of merchantability and fitness for a particular purpose and non-infringement. www.shadonline.com does not represent or warrant that the functions contained in the site will be uninterrupted or error-free, that the defects will be corrected, or that this site or the server that makes the site available are free of viruses or other harmful components. www.shadonline.com does not make any warrantees or representations regarding the use of the materials in this site in terms of their correctness, accuracy, adequacy, usefulness, timeliness, reliability or otherwise. Some states do not permit limitations or exclusions on warranties, so the above limitations may not apply to you"

The part is K0GT18ST, cost me over $65 delivered (Thanks alot SHAD!)
If you have a smaller bag, you likely have a lesser chance of the cracking. Looking at it, the washers barely covered much outside of the hole. I'd suggest adding some wider washers under the ones they supplied, to spread the load  from the bag. My uneducated guess is that too much stress is applied to the 1/8" around the hole that the supplied washer covers, resulting in a crack that eventually spreads to the outside.
I don't know if GIVI is any better, but I do know that for example, CHASE HARPER offers a lifetime warranty on their stuff, even repaired a muffler damaged bag for free.

So, the metal plate that you bolt to the luggage rack has cracks around the mounting holes?  Interesting, I will have to check mine...

Solidly on the list of things to do on Friday.

I pulled my trunk off on Friday and looked at things and all is well on mine. It has been mounted for around 4 years with no issues.

Well, owning one of the first Shad 42L that Murph sold, and having it mounted on my C10 for about 3-4 years, mounted on my custom made metal brackets, where the surface of the shad plate was directly mounted to flat metal, I had no issue... mind you I did wisely use a full size washer on the plate at install...
When I transferred this to my C14, it was undamaged, and due to the method for mounting on that rack, kept the larger washers in place, and used longer bolts to allow mounting with the existing "slots" on the C14's plastic rack, but stacked large diameter fender washers below the rack, along with helical lock washer and nuts... no issue, re installed it on my new plastic rack from Kaw, which was the recall replacement rack, in the same fashion.... still no issue today... so that's a total of almost 13 years, with no issue...
I think  maybe you should have simply used a larger washer, right from the start, and also maybe not have been so agressive in tightening the bolts,  sounds like when installed you overtightened them just a touch, dishing the metal on that plate, under the bolt head, and a crack formed.
Granted, its a heavier box than my 42, but thousands of miles, many of which my wife was leaning against this bag, and it carried 20+ plus inside for most of its life, I can say I had no issues.

You are the first person I have seen on either Concours site that has reported anything negative about this system, sorry to hear of your issue.

As you never revealed what rack surface you had tyhis mounted to, I can only assume you mounted it like I did, to the existing plastic rack, adding the larger washers should protect your investment on the "replacement" plate for a longer period of time.

I'll point out they did make an adapter kit, to insure a stronger perch mountingfor the bags, which I was not aware of, just saw it, I have to assume this was called for as so many people in COG were buying these...and asking for a stroner mounting surface...

http://www.shadonline.com/category-s/476.htm

Best of luck.  A lot of us have Shad Cases, and some have actually used the side bag mount adaptors to replace the side luggage with them. Let's see how many people report back with similar results.
Thank you for the cautionary heads up.

I don't think I over tighten them, but do
think the washers are too small. I assumed (wrongly)
that the supplied hardware would suffice.
I will use better washers.

In the mean time, I'm out of state waiting for
A tow, my rear tire got cut and is toast, in super 8
in Phoenix.
Hope that rack holds!

Good luck with all the repairs. Hope you get it all sorted quickly...