One of the hard facts of life is that if you turn a hose on in your pond to add water and walk away, you will forget it and probably kill some fish. Yeah, this is not literally true but it hopefully gets the point across that if you leave a hose running in your pond, you'll almost certainly eventually forget it and kill some fish.
Water timers are very convenient and can save our bacon when we forget the hose is still running. However, everything mechanical eventually fails including water timers. So how can we use water timers’ safety feature w/o incurring the potential disaster associated with their inevitable failure?
In the past, I have used two brands of water timers, Melnor and Dramm. Both brands have eventually failed. My personal plan has been to set a backup timer for slightly longer than the time on the water timer. When the backup timer goes off, I check the water timer to verify that it has indeed stopped the water flow. As mentioned, all my timers have eventually failed and I was glad I had a backup plan.
Another possible answer to solving this problem is to use redundancy. The concept employs the fact that even though all mechanical things eventually fail, it is very unlikely that they will all fail simultaneously. So if we put two (or more) properly-functioning water timers in series and one fails, the other(s) will likely still function properly.
I recently purchased (from Amazon) three different brands of water timers, Orbit (green), Nelson (yellow), and Gillmour (blue). BTW, of the three, only Gillmour came with any sort guarantee/warantee. It was for two years.
Above is what a 3-gang setup for redundant water timers could look like. In truth, probably two in series is enough but I had the three so I put them together. I’ll run them this way until two fail. Then I’ll try to remember to report how long they lasted.