Toilet water supply lines (WSL) can leak for a number of reasons. A typical reason is over-tightening of the locknut. The locknut is included in the package when a user purchases a fill valve (a common one is Fluidmaster). Many toilets, when originally installed in a home, are delivered to the general or plumbing contractor with the fill valve already installed in the water supply tank. This means the that the locknut was already installed and over-tightened at the factory by Kohler, American Standard, and most other manufacturers. There are usually numbers and letters in the water supply tank of significance to the manufacturer. It allows them to track the manufacturing date and delivery to the job site. When you have a water supply line failure, it is a good idea to have your insured, adjuster, or restoration contractor note the information in the water supply tank, as well as retain the evidence, including the supply line, locknut, and the entire fill valve.

Another mode of failure is inadvertent over-tightening. Some water supply lines fail to have proper installation instructions, and they merely advise the installer, who could be your homeowner, "do not over tighten". What does that mean? Does that mean hand tighten only, does it mean it not to use a pipe wrench but that you can use crescent wrench? Thus, even though your insured is the one who over-tightens the lock nut, you may be able to affect a recovery due to faulty installation instructions or design. Many of the locknuts are plastic and do not perform well when over-tightened.