Not every flight that is super cheap is a mistake fare. Sometimes, a flight is simply that cheap.
The website Seats and Stools looked to find the cheapest flight in the world, without the use of any promotional or discount codes. Their conclusion? An $11 one-way flight with Ryanair from Warsaw, Poland to Brussels, Belgium.
Seats and Stools used Google Flights and ended up searching hundreds of airlines over thousands of cities. They searched for flights 7 weeks from that date, and since they did their searches at the beginning of May 2016, the flights they found data for were the last week of June 2016.