Yes, a toilet can occasionally overflow without being flushed, though it is relatively rare. This typically happens when the fill valve or float mechanism fails mechanically, getting stuck in the open position and causing water to pour continuously into the tank and overflow tube. The water fills the bowl faster than it can drain away, leading to a spontaneous overflow. Additionally, severe municipal sewer main backups can force raw sewage upward through the drain pipes. Callout type="warning" title="Hygiene Warning": Shut off the water supply valve immediately to stop unprovoked flooding. Verification: Check that the fill valve float moves freely and shuts off water flow before reaching the overflow tube.
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