The RNG
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. It is included into the machine's software.
Random numbers are constantly generated by the RNG at an extremely high rate. The most recent random number is used to determine the result when the Play button is pressed. That means that the result depends on exactly when the game is played. The result would be different a fraction of a second earlier or later.
The numbers generated by the RNG seem to be not exactly random. The sequence of numbers is repeated by most of the RNGs. The cause of it is in poor programming. It is relatively easy to build pseudo RNGs with very long periods. The computer that is able to complete a single period in the expected lifetime of the universe hasn't been invented yet. The access to the pseudo RNG code and seed values was known to Ronald Dale Harris, a former slot machine programmer. It was he who discovered equations for specific gambling games like Keno. They allowed to predict 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 player cannot see the secquence.

Percentage Of The Payout
The percentage of winnings paid out of the money that is wagered by players is 82–98 per cent. This payout percentage is called theoretical. Jurisdictions influence the minimum theoretical payout percentage. It is typically established by law or regulation. A certain winning pattern is determined by every casino individually. It contains the information about 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 is given back to the players.
A slot machine's theoretical payout percentage is set at the factory when the software is written. Changing the payout percentage requires a physical swap of the software. It is done infrequently because this is a time-consuming process. One can find a tamper-evident seal on the EPROM in certain jurisdictions. It can only be changed in the presence of Gaming Control Board officials. Other jurisdictions randomly audit slot machines 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. After the change is made, 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.

Machines That Are Linked
Linking the machines in a special way is made to offer a particularly large prize, or jackpot. Each slot machine in the group contributes a small amount to this progressive jackpot, awarded to a player who gets a specific combination of symbols. The amount of the progressive jackpot is far higher than any single slot machine's.
Multiple machines can be sometimes linked across multiple casinos. In these cases the manufacturer owning these machines is responsible for paying the jackpot.

Near-miss Programming
Computer software controls the reel display of modern slot machines. That is why the slot machine can be programmed to display combinations that are close to winning combinations.
Such showing combinations that are similar to winning combinations more frequently than would occur randomly is called near-miss programming.
There is a related phenomenon that is also sometimes called near-miss. The chance of a winning combination appearing on a pay line is controlled by the winning percentages programmed into the slot machine. However, the combinations appearing above and below the pay line 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. Only if abbreviated physical reels are used to display a win pattern based upon the RNG this can occur. In video slot machines the symbols that appear around the winning line are usually an accurate depiction of how the reels were mathematically modeled.
There was an investigation held by the Nevada Gaming Commission concerning the issue of a near-miss above or below the pay line. It was considered to be legal as long as the near-miss above or below the pay line was not specially programmed. In other words, the near-miss must be just as likely to occur as any other combination. It is impossible to program the machine to show winning combinations more frequently than other combinations above or below the pay line.
In Australia the usage of near-miss programming, where a near miss is inaccurately displayed is also illegal. 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.
EPROM computer chips controls modern slot machines and coin acceptors have become obsolete in favor of bill acceptors. It is difficult to defraud these machines because they their bill acceptors are designed with advanced anti-cheating and anti-counterfeiting measures. An example of modern ways of slot machine defrauding is directing microwaves toward it to disrupt its proper functioning.

There exist at least two types of slot players.
Persistent player.
The players of this type believe that very soon there will be a period when the machine begins to pay. Just a second and the coins will be flying all over the casino!
Players become so persistent if they have already spent a lot of money for this slot and haven’t got anything or if they have found a slot machine that haven’t paid for a long period of time. They believe that a big win is sure to happen right now. The rush is that if they leave after depositing so much money in the machine it will erupt and the payouts will be often and large. Maybe it can really work if you have already spent thousands of dollars and have got nothing back. The machine can’t but paying.
Many years ago in the case of the payout players touch the coins to check whether he coins were hot or cold. If the coins were hot they believed that the machine had not paid for a long period of time. But in reality any machine has numerous lights which heat the coins.
The chaser.
The player believes the slot machine is not one of those machines that never pay. The player have won and being reinforced by the win gains trust in the slot machine. A sense of comfort exists between the slot machine and the player.