Random Number Generator
Modern slot machines are computerized. So the odds are whatever they are programmed to be. The reels and lever are present for historical and entertainment reasons only. The positions of the reels are chosen by a Random Number Generator. There is a Random Number Generator in the machine's software.
The RNG generates random numbers at an extremely high rate. As soon as the Play button is pressed, the most recent random number is used to determine the result. The result is different depending on exactly when the game is played. The result would be different a fraction of a second earlier or later.
The RNG was noticed to generate not exactly random numbers. The sequence of numbers is repeated by most of the RNGs. Poor programming is its reason. To build pseudo RNGs with very long periods is relatively easy. The computer that is able to complete a single period in the expected lifetime of the universe hasn't been invented yet. Ronald Dale Harris, a former slot machine programmer, was the only one who had access to the pseudo RNG code and seed values. He discovered equations for specific gambling games like Keno. Then the prediction was made that the next set of selected numbers would be based on the previous games played. However, this is impossible for most machines, because the RNG picks numbers even when the machine is not being played. So the numbers can't be exactly found in a sequence.

Payout Percentage
Slot machines usually pay out as winnings 82–98 per cent of the money that is wagered by players. This payout percentage is called theoretical. The minimum theoretical payout percentage varies among jurisdictions. Law or regulation typically establish it. There is a certain winning pattern nearly in every casino. It concerns the amounts they pay and the frequencies of these pay-outs. The main criterion for selecting the winning patterns on slot machines is that they should yield a certain fraction of the money played to the house. The rest of the money goes to the players.
When a slot machine's software is written its theoretical payout percentage is also set. To change the payout percentage after a slot machine has been placed on the gaming floor a physical swap of the software is required. Being a time-consuming process it is done not very often. Certain jurisdictions presuppose the EPROM to have a tamper-evident seal. Only Gaming Control Board officials can change it. According to other jurisdictions slot machines are randomly audited to ensure that they contain only approved software.
The Nevada Gaming Commission is working now with Las Vegas casinos on technology that would allow the casino's slot manager to change the game, the odds, and the payouts remotely. Only after the selected machine has been idle for at least four minutes can the change be done. The machine must be locked to new players for four minutes and display an on-screen message informing potential players that a change is being made after the change is made.

Linked Machines
Offering of a particularly large prize, or jackpot becomes possible when a group of machines is linked in a special way. If a player gets a specific combination of symbols the progressive jackpot from this group of machines is given to him. A single slot machine's jackpot is considerably less than the progressive jackpot.
Multiple machines can be sometimes linked across multiple casinos. In these cases, the machines may be owned by the manufacturer, who is responsible for paying the jackpot.

Near-miss Programming
Computer software controls the reel display of modern slot machines. Combinations that are close to winning combinations can be displayed on the slot machine intentionally.
The term near-miss programming is used to denote this practice.
A related phenomenon is sometimes called near-miss. Winning percentages programmed into the slot machine control the chance of a winning combination appearing on a pay line. The combinations above and below the payline are all roughly equally randomly distributed. This means it is much more likely that a winning combination will appear above or below a pay line than on the pay line. The occurance of this phenomenon is possible when abbreviated physical reels are used to display a win pattern based upon the RNG. Video slot machines have virtual reels and the symbols that appear around the winning line are usually an accurate depiction of how the reels were mathematically modeled.
The issue of a near-miss above or below the pay line was also investigated by the Nevada Gaming Commission. They ruled that this was legal, so long as the near-miss above or below the pay line was not specially programmed. That means that the possibility of near-miss occurrence should be equal to the possibility of any other combination. Winning combinations aren't supposed to be shown more frequently than other combinations above or below the pay line.
In Australia near-miss programming, where a near miss is inaccurately displayed is not allowed either. Stop motion cameras are used by regulators for manufacturer's practices audit.

Slot Machines Fraud
Sometimes mechanical slot machines and their coin acceptors seem to belong to cheating devices and other scams.
Modern slot machines are controlled by EPROM computer chips and coin acceptors have become obsolete in favor of bill acceptors. Different advanced anti-cheating and anti-counterfeiting measures have been taken to make it difficult to defraud these machines. One of the recent attempts at defrauding slot machines is directing microwaves toward it to disrupt its proper functioning.