My toilet is obnoxious. Once a week or so I have to open up the tank and screw around with the mechanism. It's not that it's worn out or broken. It's defective by design. It involves a buoyant valve in the bottom of the tank which is lifted by a chain connected to a lever that extends from the flush handle.
The first problem is the way the chain is attached to the lever. A simple metal clip goes through a hole in the arm and a ring on the end of the chain. Every so often either the chain or the lever will slip out of the clip, disconnecting the mechanism. Usually the chain will fall into the valve and prevent it from closing.
The other problem is that the chain is long enough to become hooked under the valve's hinge, jamming everything up.
If I wasn't a looser renter I'd replace the whole setup with something that works better.