Indeed it is important in this case of course that this does not happen :) To see the increased reliability and P(glue failure)<P(single failure) you need to assure the glue systems are very simple and well built -- and preferably they need to be much smaller than the system you're protecting.
Another adequate expression to apply here is
"Who watches the watchmen?"
The answer again is the watchmen must watch themselves and be very reliable.
On this topic I recommend von Neumann's (the brilliant mathematician) "Computer and the brain" book, where he explores how computing systems can be reliably interconnected and how those failure probabilities interact. He was interested on how the brain could be so robust to failure -- don't worry there's no time spent speculating on how the brain works, instead he derives from first principles properties of reliable computing components, and possible reliable designs (the brain's unknown internal workings at the time, and now to a lesser extent, would follow as a special case). He used this same approach in analyzing the principles of life, where he came up with a self-replicating machine with a tape encoding of itself, predating the discovery of DNA -- it's a very inspiring and powerful approach. Unfortunately he could not complete 'Computer and the Brain', he was in declining health due to cancer and died while writing it. What was left is still very interesting imo. He is one of those giants whose shoulders we can sit on to peek over the horizon :)