The RNG
Nowadays 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. The machine's software contains it.
The RNG generates random numbers at an extremely high rate. When a player presses the Play button the most recent random number determines the result. The result varies depending on exactly when the game is played. A fraction of a second earlier or later, and the result would be different.
The RNG was noticed to generate not exactly random numbers. The sequence of numbers is repeated by most of the RNGs. The cause of it is in poor programming. 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, had access to the pseudo RNG code and seed values. Specific gambling games equations were discovered by him. 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 player cannot see the secquence.

Percentage Of The Payout
Winnings 82–98 per cent of the money that is wagered by players are typically paid out by slot machines. The term theoretical payout percentage is used to denote it. Jurisdictions influence the minimum theoretical payout percentage. Law or regulation are used to establish it. Nearly each casino has its own winning pattern. 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 is given back to the players.
A slot machine's theoretical payout percentage is set at the factory when the software is written. To change the payout percentage after a slot machine has been placed on the gaming floor a physical swap of the software is required. It is done infrequently because this is a time-consuming process. There is sometimes a tamper-evident seal on the EPROM. To change it Gaming Control Board officials should be present. 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. 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.

Linked Machines
Linking the machines in a special way is made to offer a particularly large prize, or jackpot. 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.
In some cases multiple machines are linked across multiple casinos. In these cases, the machines may be owned by the manufacturer, who is responsible for paying the jackpot.

Near-miss
The reel display of modern slot machines is controlled by computer software. That is why the slot machine can be programmed to display combinations that are close to winning combinations.
This practice is called near-miss programming.
There is a related phenomenon which is also 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. 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.
The issue of a near-miss above or below the pay line was also investigated by the Nevada Gaming Commission. 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, any other combination must be just as likely to occur as the near-miss. The machine cannot be specially programmed to show winning combinations 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 were changed for bill acceptors. They are designed with advanced anti-cheating and anti-counterfeiting measures and are difficult to defraud. One of the recent attempts at defrauding slot machines is directing microwaves toward it to disrupt its proper functioning.

Know what you are playing.
You should know exactly how many coins you are supposed to insert into a machine. Before leaving a machine cash out your credits.
Only if you have fun your gambling won't be stressful. If you feel lucky increase your bets and vice-versa.