Systematically extracting value from promotions in ways the terms prohibit: duplicate accounts, exceeding the maximum bet while wagering, betting patterns designed to clear wagering with near-zero risk, or coordinated group play.
What operators mean by it
Duplicate or linked accounts to claim a welcome offer more than once. This is the clearest case and the easiest for them to detect through payment fingerprinting.
Exceeding the maximum bet while wagering. Almost every bonus caps the bet size during playthrough, commonly at €5. Going over it, even once, voids the bonus at most operators.
Low-risk betting to clear wagering. Covering both sides of a near-even market, or grinding a very low-volatility game purely to satisfy the multiplier with minimal variance. Terms usually describe this as low-risk or equal-bet play and prohibit it explicitly.
Coordinated group play, where several people work the same promotion together.
The honest mistakes that get caught anyway
The max-bet rule catches more well-meaning players than deliberate abusers. You are wagering a bonus, you increase your stake without thinking about the cap, and the bonus is void. There is usually no warning at the moment you place the bet, and appeals rarely succeed because the term was disclosed.
Restricted games are the second trap. Some titles contribute nothing toward wagering, and a few are banned outright while a bonus is active. Playing one can void the bonus rather than simply not counting.
How to stay clear of it without thinking hard
Read three numbers before you accept any bonus: the maximum bet during wagering, the game contribution table, and the expiry. Then stay well under the max bet rather than at it, because rounding and currency conversion can push a €5 cap over the line.
Or decline the bonus. Your deposit without a bonus carries no wagering, no max bet and no restricted games, and it is withdrawable at any time. For a lot of players that is the better deal, and it is always available.