The error can occur both ways. Or the player thinks that he has won and is outraged that the slot has not credited him with money. Or the winning combination is not visible on the screen, and the slot adds credits to the balance. It is believed that such problems are very rare and are usually common in downloadable casinos.
There are also slots programming errors. For example, he constantly gives out the wrong winnings. This happened with the Orc v’s Elf slot from Real Time Gaming . The issue was discussed on the Casinomeister forum in December 2013, right after the slot was launched.
The players received information about the winnings, but received only a part of this money or nothing at all. After it became known about the problem, the slot was temporarily removed from the casino and the error was fixed.
The casinos are NOT obligated to pay out winnings that occurred by mistake
Mistakes do happen from time to time. The casinos cannot prevent this, since the games are not created by them. Operators protect themselves from claims for the payment of erroneous winnings. This is stated in the terms and conditions of the casino. I think every casino warns in their rules that they will not pay out money won due to a game error. Below is an excerpt from the JoyCasino rules.
There have been high-profile cases where land-based casino players have won millions by mistake. Some have even sued casinos. But this is useless. Operators cannot be held responsible for developer errors. Therefore, you should not be indignant in such cases.
Another thing is that it turns out that dishonest developers can refuse to pay, hiding behind a slot error. This was the case with Sheriff Gaming, which went bankrupt a few years ago due to legal proceedings that revealed many illegal frauds of the owners of this developer.