Wikipedia
Miraclebet
Miraclebets are a particular case arising on betting markets due to either bookmakers' different opinions on event outcomes or plain errors. By placing one bet per each outcome with different betting companies, the bettor can guarantee to make a profit. As long as different Bookmakers are used for betting the Bookmakers do not have a problem with this.
A typical Miraclebet is around 5%, sometimes less, however 5%-10% are also normal and during some special events they might reach 20%.