What failure modes are there for electric water heater elements?
There's a few problems. The most common is the tank rusting out. (This can usually be prolonged if you replace the sacrificial anode regularly, but it won't prevent it forever.) The second is sediment. If you have a lot of rust in your pipes or water with a lot of sediment, it can build up in the bottom of the tank. It can also build up on the elements. 

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