The RNG
There are many modern computerized slot machines. So the odds are whatever they are programmed to be. The reels and lever are present for historical and entertainment reasons only. It is a Random Number Generator that chooses the reels positions. The machine's software contains it.
The rate of generating random numbers by the RNG is extremely high. When you pres the Play button the result is determined by the most recent random number. This means that the result varies depending 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. Most RNGs will repeat their number sequence. The cause of it is in poor programming. To build pseudo RNGs with very long periods is relatively easy. There is no computer that is able to complete a single period in the expected lifetime of the universe. It was Ronald Dale Harris, a former slot machine programmer, who knew the pseudo RNG code and seed values. 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 you can't find an exact place of a number in the sequence.

Payout Percentage
Slot machines usually pay out as winnings 82–98 per cent of the money that is wagered by players. The term theoretical payout percentage is used to denote it. The minimum theoretical payout percentage varies among jurisdictions. Law or regulation are used to establish it. 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 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. Physical swap of the software is needed for changing the payout percentage after a slot machine has been placed on the gaming floor. Being a time-consuming process it is done not very often. There is sometimes a tamper-evident seal on the EPROM. 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 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
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. 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
The reel display of modern slot machines is controlled by computer software. Combinations that are close to winning combinations can be displayed on the slot machine intentionally.
This practice 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. There are roughly equally randomly distributed combinations above and below 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.
The Nevada Gaming Commission investigated 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. 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.

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. 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.