The RNG
Nowadays slot machines are computerized. They programme the odds. In modern slot machines, the reels and lever are present for historical and entertainment reasons only. A Random Number Generator determines the positions of the reels. It is included into the machine's software.
Random numbers are constantly generated by the RNG at an extremely high rate. As soon as the Play button is pressed, the most recent random number is used to determine the result. That means that the result depends on exactly when the game is played. The result is different every fraction of a second.
It was noticed that the RNG does not actually generate random numbers. It turned out that The number sequence is repeated time after time. 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. It was Ronald Dale Harris, a former slot machine programmer, who knew the pseudo RNG code and seed values. Specific gambling games equations were discovered by him. They allowed to predict that the next set of selected numbers would be based on the previous games played. But it is impossible for most machines, because numbers are picked by the RNG even when no one plays. So you can't find an exact place of a number in the sequence.

Payout Percentage
Winnings 82–98 per cent of the money that is wagered by players are typically paid out by slot machines. This is known as the theoretical payout percentage. The minimum theoretical payout percentage varies among jurisdictions. Law or regulation are used to establish it. A certain winning pattern exists nearly in every casino. The amounts they pay and the frequencies of these pay-outs are reflected in it. The selection of the slot machines winning patterns is chosen in such a way that it could yield a certain fraction of the money played to the house. As for the rest of the money it goes 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. Being a time-consuming process it is done not very often. There is sometimes a tamper-evident seal on the EPROM. Gaming Control Board officials should be called for changing it. Other jurisdictions constantly ensure that slot machines contain only approved software.
The technology being developed by the Nevada Gaming Commission would allow the casino's slot manager to change the game, the odds, and the payouts remotely. The change cannot be done instantaneously, but only after the selected machine has been idle for at least four minutes. 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.

Machines Linking
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. The amount of the progressive jackpot is far higher than any single slot machine's.
Sometimes multiple machines form multiple casinos. In these cases the manufacturer owning these machines is responsible for paying the jackpot.

Near-miss
The reel display of modern slot machines is under computer software's control. Combinations that are close to winning combinations can be displayed on the slot machine intentionally.
This practice is called near-miss programming.
This term is also used for a related phenomenon. Winning percentages programmed into the slot machine control the chance of a winning combination appearing on a pay line. 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. This phenomenon only occurs where abbreviated physical reels are used to display a win pattern based upon the RNG. 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. They ruled that this was legal, so 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 can't show winning combinations more frequently than other combinations above or below the pay line.
Australia also forbids to use near-miss programming, where a near miss is inaccurately displayed. Stop motion cameras are used by regulators for manufacturer's practices audit.

Fraud
Sometimes mechanical slot machines and their coin acceptors seem to belong to cheating devices and other scams.
Coin acceptors have become obsolete in favor of bill acceptors and modern slot machines are controlled by EPROM computer chips. It is difficult to defraud these machines because they their bill acceptors are designed with advanced anti-cheating and anti-counterfeiting measures. Recent attempts at defrauding slot machines involve manipulating the EPROM, such as by directing microwaves toward it to disrupt its proper functioning.

You must know exactly the game you are playing.
It is important to know the correct amount of coins that should be inserted into a machine. Your credits should be cashed out before you leave the machine.
Slots shouldn't be a stress for you, just enjoy. You can increase your bets only when you are winning, and decrease your bets when you are losing.

You can be sure that slot machines are designed to be honest. What you can really doubt is the honesty of some manufactures.
In the old days, slot machines worked mechanically, with independent wheels spinning until they each stopped randomly. Today’s slot machines are light years away from those lumbering machines of old. There is a chip inside of every slot machine, this chip is programmed to select numbers randomly. The selected number determines the symbol which then comes out. There is a computer code. It generates the random numbers and determines whether the slot machine is loose or tight. The gaming inspector will examine the machine to make sure it will not deceive the player. The machine has to pay out either to the legal rate or better. Of course the latter happens very seldom.
Because slot machines don’t pay off with every spin, it’s difficult for customers to tell whether a slot machine’s chip has been programmed to pay off less often than is legally required. Before the machines are installed in casinos they are examined by inspectors. But you must understand that inspectors can’t examine every machine. They examine the brands. Once inspectors approve a brand of machine, the manufacturers can make changes to individual machines as long as the changes fall within the legal bounds. Some dishonest manufactures use the right to produce machines, but use a different chip, which is programmed to pay less or not so often.
For example, the Nevada Gaming Commission caught American Coin using chips that did not allow poker royal flushes. Or a chip that shows near misses of a jackpot. One the one hand the machine would never have a jackpot and the player would keep on playing. If you think you’re playing a machine that is not honest, you can complain. Inspectors will respond by pulling the slot machine’s chip and seeing if it is an approved chip. Manufacturers that cheat are fined or they can lose their license.