Not flushing a public toilet is considered a crime in Singapore and if you're caught flouting it, you will be given an on the spot fine of about 150 dollars.
Over the years the city of Singapore has been described by many as one of the cleanest on Earth with roads and toilets being "clean enough to eat off", which is perhaps to be expected from a city where it's illegal not to flush a public toilet.
The reason why toilets in Singapore are so insanely clean can be traced back to the work of Lee Kuan Yew, Singapore's first and arguably most popular prime minister. Kuan Yew rose to power in 1959 and continued to serve as Singapore's leader for 31 years until he decided to step down in 1990. When Singapore became an independent nation in 1965, Kuan Yew is noted as being instrumental to the small city-state being able to so quickly transform itself from being a "poor port from the bottom rungs of the third world" to being one of the most profitable and prosperous economies on the planet.
By far Kuan Yew's most infamous policies though were his incredibly strict rules in regards to public cleanliness, most if not all of which carry hefty fines if you're caught breaking them. For example, not flushing a public toilet is considered a crime in Singapore and if you're caught flouting it, you will be given an on the spot fine of about 150 dollars, more if you're a repeat offender. Likewise, littering carries an equally heavy fine of about 300 dollars or more, depending on the size of the item. Smaller items like candy wrappers usually incur a lesser fine, whilst things like soda cans can net you a trip to court and even a caning if you're caught.